


What You Don't Know

by Kassykins



Category: Undertale (Video Game)
Genre: Adopted Sibling Relationship, Gen, Not a Love Story, Safe For Work, later chapters get kind of dark though
Language: English
Status: Completed
Published: 2016-07-11
Updated: 2016-08-07
Packaged: 2018-07-22 13:07:26
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 10
Words: 32,351
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7440427
Author URL: https://archiveofourown.org/users/Kassykins/pseuds/Kassykins
Summary: <blockquote class="userstuff">
              <p>Things underground are stagnating, and Sans is stagnating with them: days and nights pass by with nothing to show for them, and it's getting harder to keep that trademark smile plastered on his face. He's not the only one in a rut - the other scientists at the lab are losing heart, as their research comes to one dead-end after another. Dr Gaster especially is feeling the pressure after the 'failure' of his Core project to create enough energy to break the barrier. At the end of yet another unremarkable day, there's an unexpected knock on Sans' front door, one that will change his life as he knows it. As the test results start to change, hope starts to return to the scientists. As the tests themselves start to change, Sans has to question the price of that hope. Under pressure to save the underground, how far will Dr Gaster go to get the results he needs? When a line is drawn in the sand, will Sans be able to see the greater good, or will he do whatever it takes to protect his own little world?</p>
            </blockquote>





	1. It's Cold in Snowdin

**Author's Note:**

> This is my first work on this particular site. I hope you find something in it to enjoy!

It started as just a regular day, as all other things typically do. One could hesitate to call it a 'day', even, what with no sun in the sky, no seasons, no weather or natural light or wind or fresh air or... well, it was just another regular day, according to the clock. God only knows how long it had been since the last 'thing' to really happen. If any given monster squinted back into their memory, they could probably recall a 'thing' or two in the somewhat recent past - prince Asriel being born, for example, or the Core finally being turned on - but no-one could say exactly how long ago it had been or why it was so important as to qualify as a 'thing'. Time passed, completely ambivalent to who did or didn't notice it.

Get up, have breakfast, get dressed, go to work, same old same old. It was probably obvious that Sans was is a rut as time passed him by: while he loved his job at the lab, their research had stalled somewhat recently – the geothermal energy harnessed by the Core was proving adept at providing electricity enough for the entire underground, but it was nothing compared to the impenetrable energy of the barrier. Hell, if they could have found a way to use the barrier to create electricity, all their problems would have been solved! The stagnating research into the barrier had been taking its toll on the teams morale: every test came back negative, or else the numbers were so ridiculously high and erratic that they had no hope in hell of ever replicating them. The King wasn't the type to pressure people for results, but it was his money they were spending, and people kept asking...

After yet another fruitless day, Sans felt even more disheartened than usual, stopping at home just long enough to change out of his shirt and tie before heading to Grillby's. Grillby always teased him about the way he changed into his civvies the second he got home from work, and truth be told he was starting to wonder why he bothered. It was just clothes, who cares if they're a little dirty from work. Would work even care if he wore a t-shirt and slippers? They might not even notice. Probably. Wearing pyjamas to work sounded like a good plan to him, but it might be a step too far, really. God, he was tired... Piping hot dinner in hand, Sans paid the bill and headed home. He wasn't sure if he was hungry or not. He had last eaten at breakfast... hadn't he? He remembered there being breakfast, at least. Had he eaten it?

  
He only looked up as he got to the door of his house, fiddling around for his keys. Hearing a familiar sound, he looked around, feeling his spirit lift for the first time today – Papyrus bounded towards him, book bag slung over his shoulder as he ran home from school. He had just had a growth spurt, which left him looking gangly and awkward, sleeves and trousers just too short for him, and he wasn't yet used to how long his legs had become, kicking up snow as he ran.

  
“SANS!” he greeted, holding up a tatty old book excitedly “I got a new puzzle book! The librarby just gave it to me! Isn't that neat?!”

  
“That's great, bro.” Sans agreed.

  
“What's for dinner?”

  
“I bought spaghetti.”

  
Papyrus stopped before him, swaying awkwardly for a second: his hands and feet had also grown, and as such were far too large for his spindly limbs, throwing off his balance.

  
“Sounds good.” he agreed “I like spaghetti.”

  
“I know you do.”

  
Sans unlocked the door, leading them in. Papyrus immediately threw his bag on the sofa and went to the kitchen to get the plates and forks. Seeing the breakfast dishes still on the side, he took a moment to wash them up. Papyrus was a good boy, and the way he was always smiling and so full of energy... it made it easier to be broke, to support his little brother, to be the responsible one, to get up in the morning... Papyrus' smile made things easier for Sans.

  
The younger brother talked non-stop as they ate dinner, telling Sans about what he had learned at school. When he noticed Sans picking disinterestedly at his food, he asked what was wrong, reminding him to eat properly. After dinner, Papyrus did his homework as Sans stared at the television - it could hardly be called 'watching', as he couldn't say with any certainty what was on, despite not having turned the channel for several hours. Sans was so tired, he just wanted to go to bed. Forever. But he made himself stay up, at least long enough to read Papyrus his bedtime story: he knew he was too old for stories, but it was one of his few joys, and as long as Papyrus didn't mind...

  
It was pretty late when the knock on the door came. It was Dogamy and Dogaressa, those dog brigade members who had just started dating (which everyone in the entire underground knew, as they could hardly keep their hands off of each other in public). Needless to say, Sans was surprised to see them: he was a scientist, he didn't have anything to do with the Royal Guard. They were polite, but clearly tense and confused.

  
“You're that scientist, right?” Dogamy asked.

  
“(The smart one.)” Dogaressa clarified.

“I-I work at the lab, yes.” Sans confirmed unsurely “Is everything okay?”

  
The dogs looked at each other a moment, beckoning Sans closer as they leaned down. Taking a conspiratorial look around, they whispered.

  
“We found something.”

* * *

 

'Something'? Sans wasn't overly happy at how vague that was, but after telling Papyrus to lock the door and not answer it to anyone but him, he dutifully followed the Royal Guard members out of Snowdin. He grew increasingly uncomfortable the further they went: the only thing in this direction was the ruins, and a whole bunch of bottomless cliffs. What could they possibly have found all the way out here? They were joined by two other dog brigade members – the Greater and Lesser dogs (weird names...) – with the last (the one who couldn't see right) standing at the ruins doors like he was a bouncer and it was Friday night at Grillby's. Was whatever they found serious enough that they needed all these Royal Guard members to attend? Sans zipped up his jacket. He felt vulnerable. There was a reason Mr 1HP was a scientist.

  
Luckily (perhaps), what they had found wasn't too far from the ruins entrance. On the ground, smeared on one of the old puzzles, was an odd red liquid. It was thick, drying brown, and smelled oddly metallic (no doubt the smell being what had attracted the dogs in the first place). The dog brigade were stumped, never having seen anything like it, but Sans had an inkling as to what it was. He had read about 'blood', and knew it only came from humans. An injured human... didn't the King and Queen have a human child? This could be bad. They split up to search, Sans hanging back, really not wanting to get involved any more. Greater Dog was the one to find the human, hiding behind a pillar – it was curled up into a ball, shaking and terrified. Poor thing... it wasn't even as big as Sans, its leg clearly broken as they cradled it carefully.

  
“What is it?” Dogamy asked, poking his head around the pillar.

  
“(It smells weird).” Dogaressa agreed.

  
“It's a child.” Sans told them “A human child.”

  
“What's all this red stuff?”

  
“(It tastes weird).”

  
“Don't lick it.” Sans scolded, squatting down an arms reach away from the newcomer.

  
It had been a while since Sans had glimpsed that human child the King and Queen had, but he was sure it was bigger. He sighed deeply: he just wanted to go home. He didn't want to deal with this. He wanted to go to bed. However, if the King and Queen found out that he left the kid here, he'd be in trouble. Making himself as friendly looking as possible, he held out his hand to the human.

  
“Hey, kid.” he greeted “Are you okay? That looks like it hurts. Do you need any help?

”  
The child flinched violently, curling up even tighter. It had clearly heard him. It tried to crawl further into the corner, blocked by the solid stone wall.

  
“Should we fish it out?” Dogamy asked.

  
“No, if we scare it, it might hurt itself.” Sans thought.

He sighed, thinking about Papyrus all alone at home. Hitching up his trousers, he sat down on the ground, still facing the human. What should he do? He wasn't a biologist, nor a parent. Looking at it... well, when Papyrus had been that size, he had been 8 or 9 years old. Although he had always been on the tall side. As he thought, he absently stuck his hands in his pockets, startling when his hand brushed something unexpected. What was that? Pulling it out, he smiled: it was one of those astronaut food bars, which Papyrus had stuck a post-it note to. 'HAVE A GREAT DAY! =D Papyrus', it read.

  
Sans stopped smiling, jumping clear out of his... well, he didn't have skin, but if he did, he would have leapt clear out of it as he was launched at from behind the pillar. Luckily for him, it was the food that the human was after, sharp little teeth tearing into it when the wrapper was barely off. In a rare moment of quickness, Sans grabbed it, but it didn't seem to mind too much: it was more hungry than anything. How long had it been down here before the dogs had sniffed it out? Although, that blood was still pretty fresh...

  
Sans tried to pick the kid up so that he could stand, but it was only a little smaller than he was, making it next to impossible. The dogs caught on pretty quickly that food was the key, plying the kid with whatever they had in their pockets and some random sweets that they had found as Sans took a look at its leg. Being a skeleton, he had a certain knowledge where bones were concerned, but you didn't need to be an expert to be able to see the humans leg was pretty badly broken. He set it as best he could with the things at hand, the human mutely smacking him on the shoulder and head when it hurt. Weird... the human was completely silent, aside from a few whimpers, despite the fact it was clearly scared and in pain. It had a hell of a squint, too. Did it need glasses?

  
“(What do we do with it?)” Dogaressa wondered as the brigade made its way back to Snowdin “(It's too late to take it to the palace).”

  
“Yeah, it's best to go in the morning.” Sans agreed “The King and Queen will know what to do with it.”

  
“You okay?” Dogamy asked him “It looks heavy.”

  
Indeed, Sans was struggling under the humans weight as he carried it piggy-back style back to town, the child only growing heavier after it fell asleep.

  
“Human's physical matter is a bit denser than ours.” he admitted, hoping one of the Royal Guards would take the burden.

  
The hint was lost on them, however, as Dogamy and Dogaressa were too interested in each other, and the others didn't seem to be particularly bright... After what seemed like a mammoth walk, they finally reached home. Were they going to deposit the kid in the inn? Was one of the dogs going to take it for the night? Apparently not.

  
“It's really good of you to take the human in.” Dogamy complimented “I wouldn't feel safe with it in my house.”

“Y-yeah...”

“(Especially with it leaking red stuff like that.)” Dogaressa agreed “(You're pretty brave!)”

Sans wasn't brave, he was tired and didn't have the energy to argue. The human was tiny and had poor eyesight, it was unlikely to be any threat, especially injured. He thought about how Papyrus would handle their guest, but he was pretty good with younger kids. There couldn't be that big a difference between human and monster kids... The dogs promised to return in the morning, but Sans wasn't going to hold his breath. He sighed when he realised he couldn't get his keys without letting go of the kid, calling for his brother to open the door.

* * *

 

Sans woke up with a burden. He had a thing he had to do today. Even though he logically knew it was a simple enough thing, it felt like the weight of the world was on him. He'd drop the kid off with the King and Queen, and it wouldn't be his problem any more. He'd have to drop by the lab to tell Dr Gaster where he was going, but he was sure the boss would understand. Papyrus was awake first, of course, but Sans wasn't sure where he was, as his room was empty and there was nothing but silence in the kitchen. Had he gone to school early? It wouldn't have been the first time.  
Once he got to the landing, Sans looked over the banister to the sofa, where he left the human sleeping the night before. He startled when he saw the kid was gone, causing him to bolt down the stairs, nearly tripping. Did it escape?! Did Papyrus let it out? It was injured, it couldn't get far! Being short had some advantages, and as Sans ran for the door, he managed to spot something under the table: all wrapped up in the blanket the brothers had left for it, the human was fast asleep, fit snugly under the coffee table. Well, at least it hadn't gotten out. Sans sighed, feeling his heartbeat return to normal. At least now he had time for a cup of coffee.

  
The poor thing must be terrified, to hide under the table like that. Sans had never seen the surface himself, he couldn't imagine it was that much scarier down here. Still, it was just a kid. What if the situation had been reversed, and it had been Papyrus on the surface alone? The thought stayed his hand a moment... the kid would definitely be hungry, but there wasn't much food in the house. He startled as the front door clacked open, Papyrus hurrying in and closing it behind him.

  
“Oh, you're up!” he declared, surprised “I was worried I wouldn't have time to wake you properly.”

  
“Where did you go so early?” Sans asked.

“To get breakfast!”

Papyrus pulled a steaming package from his book bag, laying it on top of the table.

“Fresh cinnamon bunnies!” he announced “They're super scrummy yummy!”

He immediately dropped to the ground, looking under the table, and grabbed one of the buns.

“Human!” he called “Are you awake now? Remember me? Papyrus? I bought you breakfast!”

“Good thinking, bro.” Sans complimented, coffee in hand as he shuffled back into the living room.

“Human!” Papyrus tried to tempt, holding the bun out to them “Wakey wakey.”

“Don't force the kid.” Sans recommended “They had a long day yesterday. You ready for school?”

“Almost. Watch the human, okay?”

Sans chuckled and Papyrus leapt up, grabbing another cinnamon bunny and shoving it in his mouth before running up the stairs. Hearing a shuffling under the table, Sans put his coffee on top and leant down to take a look: the cinnamon bun Papyrus had left had disappeared, the blanket shuffling slightly.

“So you are awake.”

The lump under the blanket flinched. Sans stayed quiet as the human carefully unwrapped itself from its fabric cocoon, peering at him uncertainly. It seemed to recognise him from yesterday, cautiously emerging with the bun still in it's mouth.

“How are you feeling? Does your leg hurt?”

The human gestured with its hands. It took a second for Sans to realise: it was sign language. The kid was telling him it was thirsty. Sans got to his feet, going back to the kitchen, and got a glass of water, which the kid gratefully downed before shoving another bun into its mouth.

“Hungry, aren't you?” Sans noticed “More water?”

The kid drank three glasses before Papyrus came back down. The kid startled at the sight of him, grabbing Sans shirt, but calmed down soon enough with the application of more cinnamon bunnies. The kid sure was hungry. And thin. It's clothes were baggy and tatty, and some of its injuries looked older than that broken leg. Papyrus seemed not to notice, making friends with it immediately.

“Sans, the human's not dressed for the cold!” he did notice “They'll freeze before they get to the capital!”

“I'll lend them a jacket.” Sans promised.

* * *

 

Papyrus bid them both an excited goodbye as he left for school. Headed for Waterfall, the kid got the first echo flower before the pain in their leg became too much to hobble any further. Unfortunately, since they had eaten, they were even heavier than yesterday, so carrying them to the capital was out of the question. Leaning on Sans, they managed to make it back home, but was practically in tears from the effort of it.  
Well, time for plan B – Sans called work, knowing that Gaster spoke often to Asgore and hoping that the two of them would be more willing to deal with it. Unfortunately, judging by Alphys' stuttering as she answered the phone, Dr Gaster was in a bad mood.

“Oh n-no, I'm not telling him!” she insisted “You t-tell him! He l-likes y-you more!”

“I guarantee that's not true.” Sans disagreed.

However, she was already handing over the phone. Before he had even bought it to his mouth, Sans heard Dr Gaster sigh irritably.

“Sans.” he greeted coldly “Why aren't you at work?”

“Boss,” Sans reported “I'm sorry, but something happened last night.”

“Well, this had better be good.”

Keeping one eye on the kid, who was busy rebuilding their blanket cocoon on the sofa, Sans quickly relayed the story of their mysterious appearance. Dr. Gaster listened silently, but Sans could hear him grinding his teeth as his tale ended. There was a tense silence as the good doctor came to a decision.

“You did the right thing.” he begrudgingly admitted “That royal guard is so lazy, they can't be trusted to follow through on anything. I'll let the King know what you found: take care of the human until someone shows up to get it.”

“R-right.”

Orders given, Dr Gaster hung up. Sans sighed – that could have gone a lot worse, so at least there was that. Well, what now? Children were children, regardless or species, right? They didn't look that much younger than Papyrus.

“Hey, kid,” he said tentatively “You like stories?”

* * *

 

Good news: the human liked storybooks. Bad news: the longer Sans sat next to them on the sofa reading, the more he became aware of how bad they smelled. Half way through 'Fluffy Bunny', it became too bad for him to ignore, promising the end of the story after a bath. The kid didn't argue – in fact, they seemed pretty excited about the idea. Sans had to rig up some kind of hoist to keep their wrapped-up leg above the water, but at least after the bath he'd be able to put a better splint on.

  
After throwing a couple of Papyrus' bath toys in with it, Sans took its clothes downstairs (god, they smelled!) and threw them in the washing machine. They didn't own a dryer, so the clothes wouldn't be ready to wear for some time: there had to be something around here that would fit them. Keeping an ear out as he looked through his and Papyrus' closets, he heard the kid laugh a couple of times as it splashed around in the water: so it could make some noises. After 20 minutes or so, Sans stuck his head in the bathroom.

“Water cold yet?” he asked.

The kid made an embarrassed, somewhat scandalised noise, followed by the sound of water splashing indignantly.

“Don't worry, I'm not looking.” he promised “All washed up? Clean your hair?”

The kid made a noise of confirmation.

“Okay, time to get out.”  
Closing his eyes to save the kids modesty, Sans held up the towel and stepped in. The kids leg was still broken, and it used Sans arms as an anchor as it pulled itself out. Feeling it fall against him, he wrapped the towel around it and opened his eyes again.

  
“Hey, there was a kid under all that dirt!” he teased “You missed a spot.”

  
Grabbing the flannel from the side, he dipped it into the (almost brown) soapy water and ran it over the kids face. Yeah, yeah, he was that kind of big brother, so sue him. He was used to Papyrus flailing away and saying he wasn't baby bones anymore – especially since he got old enough to take baths on his own – that he expected the same reaction. Instead, the kid smiled at him, giggling a little. In spite of himself, he couldn't help but smile back.

* * *

 

When Papyrus arrived home from school, he was excited to find the human still there, not at all upset to find it decked out in his old clothes. He even brought his action figures down from his room for it to play with, sitting on the floor with it as the tv played in the background. Sans hoped he was going to be reimbursed for the humans food as he paid for the takeaway: he could afford to feed another person, especially if the kid usually ate as much as it did this morning. When he returned home, Papyrus and the human were both sat on the sofa, leant over a book.

“Sharing some puzzles?” Sans asked.

“It's a puzzle the human gave me!” Papyrus replied excitedly.

“Oh yeah?”

“Yes, I have to figure out their names based on clues they gave me!”

“What's the clues?”

The kid signed at him, spelling out its name: 'Frisk'. Sans was torn between telling Papyrus the answer and letting him figure it out himself. Both of them seemed to be enjoying the game, so he left them to it as he got the plates.

“Nyeh heh heh, I won't be beaten at puzzles!”

Frisk laughed.

* * *

 

Papyrus really was good with younger kids: after dinner, he made a fort under the coffee table with blankets and pillows and entertained Frisk with stories and puzzles until the human fell asleep.

“Should we leave it here?” he asked quietly.

“Seemed happy enough last night.” Sans figured “Looks warm enough.”

Papyrus yawned widely: he'd been busy tonight, and Sans was a little bit proud of him. Well, more than usual, anyway.

  
“Gotta finished my homework.” he declared, rubbing his eye socket “I will be victorious over quadratic formulas tonight!”

“Knock-em dead, bro.”

“Nyeh heh!”

Sans took another sip of his coffee as he watched Papyrus grab his book bag and go upstairs, yawning again. Not having done much today, he wasn't too tired, which was weird for him, so he settled back onto the sofa with a book, turning the sound on the tv down. After a while, Papyrus opened his bedroom door to request his usual bed-time story, after which Sans returned downstairs to wash up the plates and cutlery before turning in himself. As he turned the water off, he heard a quiet noise, almost inaudible but for the silence of the rest of the house: a tiny whimper. Was the kid awake? Silently, Sans crept back over and looked under the table: the kid had curled up into a ball and was shaking again, sniffing and occasionally whimpering. A bad dream? Poor kid.

Feeling his heart strings pull, Sans reached out and touched Frisk gently on the shoulder. The kid flinched violently, like they'd been shocked, which was apparently enough to wake them up. Frisk looked at him cautiously, like they were trying to figure him out, before slowly uncurling. Sure enough, the kid had been crying.

“You okay?” Sans asked “Bad dream?”

Being careful not to smack their head on the low table, Frisk crawled out, putting their arms around Sans. Not able to resist a child in need of a hug, he returned the gesture. Almost immediately, the kid was asleep again. Well... the dishes could wait until tomorrow.


	2. How Things Work

First thing in the morning, just after Papyrus had gone to school, there was an angry thundering on the door. Frisk immediately hid behind Sans, who reluctantly edged to the door to answer it. A royal guardsman stood there in their full armour, clearly a ball of tension, magic spear in one hand.

“Y-yes?” Sans greeted.

“You got the Human?” they asked, voice masked by the metal helmet.

Sans shuffled a little, pointing to Frisk, who was still hiding, clutching his coat.

“What the hell, it's tiny!” the guardsman swore, spear immediately disappearing “I gotta escort this? Damn!”

They pulled off their helmet in frustration, almost stamping on it in their anger. Sans didn't feel that much less intimidated by the young blue-skinned, red-haired fish woman he found beneath, especially as she had an impressive set of mandibles and a look on her face like she wanted to bite something.

“Those dogs should've done this yesterday!” she went on “I swear, they gave me this junk job just because I'm the new guy... when I'm head of the royal guard this kind of laziness won't be tolerated!”

“What's the rush?” Sans questioned “It's not like anybody's going anywhere.”

The guard all but growled at him. Thinking better of herself, she huffed and shifted about where she stood.

“I'm Undyne.” she greeted “The King sent me to escort the human to the lab.”

“Hm? Why the lab?”

“The Queen's idea.” Undyne reported, laughing a little “She thinks the capital is a little too far for a child to go on their own. They're going to meet us there.”

“Fair enough.”

“I don't see why you couldn't have done it alone, though.” she muttered under her breath.

She saw soon enough: while Frisk did their best to hide behind Sans as he locked the door, Undyne spotted the makeshift splint (cobbled together from two halves of a broom handle and a whole load of dishclothes), having a moment of moral dilemma before picking the child up, grumbling under her breath. Sans found Undyne amusing for about 30 seconds: she had great long legs, which she used to walk a million miles an hour, making short little Sans practically run after her. Not being at the peak of fitness, he was sure he hated her by the time they got to the first puzzle: fortunately, she seemed to have zero patience for them, using said powerful legs to simply leap over the water, or just walk through if shallow enough, grabbing Sans by the arm and hoisting him over too.

There was something to be said for strong women, but god damn. Undyne was definitely more than Sans could handle, and by the time they finally got to Hotland, he was completely exhausted. She stopped at the bridge between Waterfall and Hotland, sweating profusely in her armour.

“Ugh, damn this noise.” she groaned, squatting down to put Frisk on the ground “I hate Hotland.”

“We could probably walk alone from here.” Sans offered, happy to have an excuse to be rid of her “The lab is just over the bridge.”

“No,” she grumbled “I'll see it through. I'm a professional, not some punk kid!”

Well, somebody had something to prove. Despite her words, she was clearly suffering, bent double and running rivers. 

“You might boil in that tin can you're wearing.” Sans warned.

“Shaddup, nerd! I said I'd do it, and I'm gonna do it!”

She stood suddenly, startling both Sans and Frisk, and tore off her breastplate, throwing it aside dramatically. Sans had to stop himself snickering at her surprisingly noodley arms. Undyne stretched, recomposing herself, and pulled back her short hair.

“Okay.” she decided “Let's so this! Final stretch!”

She picked Frisk up again, stomping uncomfortably towards the lab. Dr Gaster, somehow knowing when to expect them, was waiting at the entrance, hands poised at the fingertips in that elegant way of his, just watching them approach. Undyne practically threw Frisk at him, screamed something about her job being done, and headed for the nearest cooling body of water. Dr. Gaster was less than impressed, glancing at a smiling Frisk for a second before handing them back to Sans.

“The King is a little delayed.” he reported “You're in charge of the human until he arrives. I expect you to catch up on the work you missed yesterday.”

“Sure, wouldn't want to miss out on all that nothing and failure.”

Gaster raised an eyebrow, but said nothing, stepping aside to let them in.

Morale was about as high as usual, it seemed, a few of his colleagues shuffling around the break room getting coffee as he arrived, bidding him good morning as he hung up his blue coat and picked up his white one.

“So, that a human, huh?” Dr Head mentioned.

“Seems like it.” Sans confirmed.

“I thought they'd be bigger.”

“Oh, hi Sans.” Dr Hive greeted as they exited the lift “Alphys wants you. Didn't ask why.”

“Probably about the recalibration data.” he thought, sighing “Okay, I'm on my way. She in her nest?”

“Where else?”

Sans and Frisk entered the lift, the kid looking all around interestedly. Sans was used to the lab, but he could remember it looked pretty fantastic the first time he saw it, with all its blinking machines and brightly coloured chemicals and monsters in white coats walking around looking at clipboards. He held Frisks hand to stop them wandering off as they marvelled at the things around them.

“Morning, Sans." Dr Odin called "Good day off?”

“Feels like I never left. Are we even sure it's morning?”

“Haha, tell me about it! I think Alphys wants you, by the way.”

“So I hear. Maybe she's finally succumbing to my charms, eh?”

Sans left the main lab, pulling Frisk down the hallway. Thinking about it, there was no better place to keep Frisk than in Alphys' nest, away from the dangerous chemicals. Of course, her nest was full of machine parts and wires, but Sans was sure she'd be happy to put some cartoons on for the kid.

“Watch your step.” he suggested.

Her mess was starting to spread up the stairs, as machine parts, LED's and boxes of bolts made them dangerous to traverse. With Frisks leg already in a bad way, Sans almost carried them down the last few steps.

“Alphys!” Sans called into the gloom.

There was a shuffling amongst the mess. The only real light was coming from the myriad computer monitors, the eerie blue glow illuminating the bowls, figurines, machine parts and garbage all around, the only two clear areas being an Alphys sized space on the bed and around what Sans could only suppose was a fridge being built on one wall. Frisk jumped and grabbed Sans arm as Alphys emerged suddenly from under a pile of machine parts, holding up a tiny cog in triumph.

“Victory!” she declared “Oh, hey Sans. When did you get here?”

“Your eyesight's going to get worse.” he warned her “Turn on the room light, will ya?”

“Honestly, Sans, you're always so... who's that?”

Finally noticing Frisk, Alphys went a little pale. Sans had forgotten that she wasn't particularly fond of strangers.

“Oh, this is that human I was telling you about.” he confessed “Name's Frisk.”

“W-what!?” she cried, flustering theatrically “A-A-And you bought them here?! The place is a mess! I haven't showered, I'm barely dressed! I'm not ready!”

She dived back into her pile of machine parts, making Frisk laugh. At least they weren't scared of her – although, it was pretty hard to be scared of Alphys.

“This is my buddy, Alphys.” Sans introduced “She's a robotics engineer: she built a lot of the blinky machines and doo-dads around here. Her output's pretty shocking.”

Frisk snorted in laughter. Sans wasn't entirely sure why, until he heard Alphys groan, appearing cautiously over her junk pile.

“Tell me that wasn't a pun.” she said.

“Pun?... Oh! I didn't realise!” Sans laughed, making Frisk chuckle “But cut me some slack, Alphys, it's easily done: I'm not the brightest bulb in the box, and your work is simply electrifying, I can't help but want to illuminate the kid-”

“Enough!”

Alphys finally emerged properly from her pile, straightening up her lab-coat and glasses and making sure there was no grease on her scales.

“I swear, for a quantum physicist, you'd think you'd have better jokes.”

“What can I say, I'm a fan of the classics. You wanted me?”

“I did? Oh yeah, the data!”

Wading out of her pile of machine parts, Alphys went over to her desk, pulling a stack of paper off and handing it to Sans.

“Here you go. Dr Gaster is in a bad mood still, so I'm gonna need your formulas back ASAP. Can you get it to me by the end of the day?”

“Gee, I'd love to help you with that, but I'm gonna have my hands full with the kid. If only there was some place I could leave them, some place cosy and out of the way where they could watch some cartoons-”

“Ex-cuse you!” Alphys cried indignantly “They are not 'cartoons'! It's anime!”

“Oh, my apologies.” Sans teased.

Alphys gave him a scolding look, grumbling, but she wanted her formulas.

“Okay, fine, I'll set them up here.” she agreed “But you're not leaving until I get my numbers back!”

“Speaking of leaving, when was the last time you went home?” Sans asked, looking at the futon and the piles of dirty clothes and dishes “Do you even have one anymore?”

“I think we both know that I don't.” she admitted “You know how time gets away from me. An all-nighter here, a week sleeping under my desk there, and the next thing I know I haven't been home in six months and my landlord's really mad at me for some reason. It was a real hassle.”

“You didn't pay your rent, did you?” Sans knew.

“I meant to!” she defended immediately, blushing from ear to ear “I just... forgot... A-anyway, this is way more convenient! I can just concentrate on my work now!”

“You need a hobby.”

“I have Mew Mew...”

“You need a different hobby. What happened to that ghost friend of yours?”

“Metta? She's... around...”

“Sans!” Dr Gaster called down the stairs “Do you have the data? Get to work!”

Sans and Alphys shared a look, sighing.

“Duty calls.” he said, turning to Frisk “Be good okay? I'll bring you some lunch later.”

“Don't worry, we'll have tons of fun.” Alphys promised “We'll watch Mew Mew, read some manga-”

“Get on with your work as well!” Dr Gaster called again, evidently having heard them perfectly, making them both flinch.

“Yeah, eyes on the prize, ya pervert.” Sans teased, turning to leave.

“Please stop calling me a pervert.” she sighed “It's not very nice.”

“I'll stop calling you a pervert when I find at least one person in the entire underground you haven't had a crush on.”

“There are plenty of people!” she defended.

“Oh yeah? Name one.”

Alphys opened her mouth to reply, but as Dr Gaster was still waiting at the top of the stairs, she instead settled on glaring at him for a second before stomping back into her nest.

“Leaving the human with Alphys?” Gaster figured “Out of the way, plenty of distractions... good idea.”

“Thanks, boss.”

“I want those numbers on my desk by the end of the day.”

“Right...”

* * *

 

Sans didn't like leaving Papyrus alone at night – he was old enough to be trusted alone, of course, but young enough that it still made Sans worry. Papyrus never slept well, and if Sans wasn't there to tuck him in he then was liable not to sleep at all. He always tried to get home by dinner time, but the sheer amount of data he had to sort through today made it impossible. Long after everyone else had gone home, Sans still sat at his desk, pen in one hand and coffee in the other, pouring over the numbers in the quiet, harshly lit back office.

Dr Gaster's gliding footsteps could be heard down the hall. Much like Alphys, he was a man who never went home, for better or worse, and while his devotion to his work was admirable, it also made him difficult to talk to. Sans was startled when a slender hand was laid gently on his shoulder, Dr Gaster leaning over him.

“It's late.” he pointed out kindly “Go home.”

“Oh?” Sans replied, exhaustion clear in his voice “I haven't finished yet.”

“It's fine: you've worked hard today.” Gaster insisted “And you've got your little brother at home alone, don't you? You can pick this up tomorrow.”

“Yeah... thanks.”

Sans stretched, bones clicking, and got up from his chair.

“I feel like I'm forgetting something.” he thought.

“The human?” Gaster suggested, laughing.

“Oh my god, Frisk!”

“Asleep.” the doctor informed him “I went to check on Alphys' work about an hour ago and found them both sleeping in front of the television. I took a picture.”

Sure enough, he pulled out his phone to show it off.

“It's cute. You want one?”

“Sure.” Sans laughed “You're in a better mood.”

“I watched some amusing cat videos online.” Gaster confessed “That one where the kitten sneezes and they all run away... gets me every time.”

“Whatever works for you. What happened to the King?”

“He's not feeling very well. Said something about a pie... sounds like food poisoning to me.”

“Harsh.”

“Yes, well, you'll just have to take the human home for the night.”

“All the way back to Snowdin? With a broken leg?”

“Don't worry, I've already called the Riverperson for you. They should be there by the time we get down to the river.”

Gaster led Sans back down the stairs to Alphys' nest, long legs easily stepping over the mess, and flicked on the overhead light in the most unceremonious manner. Alphys, who had been blissfully sleeping, leaning against Frisk, choked a little as she was startled awake.

“I'm up, I'm up!” she flustered “I was just resting my eyes. I'll have the results in an hour, I swear!”

Blinking a few times as she woke up properly, Alphys adjusted her glasses, blushing awkwardly.

“B-Boss...” she greeted “Sans.”

“Alphys dear, go wash your face.” Gaster ordered “I've ordered us a pizza. After dinner, I want to discuss a new project with you.”

“A new...? Oh! Sure, sounds fun!”

Alphys slapped her face to wake herself up, smiling self-consciously.

“I'll go wash my face...”

Gaster stepped past her, easily picking up the still sleeping Frisk, and headed back up the stairs.

“Oh, Sans.” Alphys said as he followed “I've thought of one!”

“One what?”

“You!” she replied triumphantly “I've never had a crush on you!”

“Oh, Alphie, we both know that's not true.” Sans teased “After all, how could you resist all of this? I'm a towering example of skeleton-manhood, a pillar of my community, with a luscious full head of hair-!”

“SANS!” Gaster scolded from the top of the stairs.

Alphys, however, was laughing.

“See you tomorrow, bonehead.”

 

 


	3. Where to Take Readings

The Riverperson had made fun of Sans for walking all the way to Hotland with the injured Frisk, an insult which stung all the more for it's accuracy. When they got home, Papyrus was still up, making a massive pile of sandwiches, and was more than happy to see Frisk had returned.

“I got a book of children's puzzles from the librarby!” he declared “We can do them together after dinner!”

Sans was too tired to do anything but watch them, but that was entertaining enough as they played on the floor with Papyrus' action figures. Exhausted, he closed his eyes for a moment, instantly falling asleep, which he only realised when he woke up to a silent, still house. Well, that was the least restful sleep of his life. He got up, back aching from having slept on the sofa, and headed upstairs. He ducked his head into Papyrus' room, silently opening the door. Papyrus was having one of his power naps, and Sans could see Frisk's brown hair poking out of the top of the covers. Well, this was better than them sleeping under the coffee table, so he turned to leave the pair to their sleepover.

“Don't...”

Sans froze. That was not Papyrus' voice. It was higher, gentler. Sans turned back, seeing Frisk shuffle about in their sleep.

“C-cole...”

'Cole'? That wasn't a word. Sans crept over, leaning over to get a look at the kid. Frisk was shaking, shuffling uncomfortably in their sleep.

“Mmmm...” they moaned “Dar... don't lie...”

Sans was so busy concentrating on what Frisk was saying, he didn't notice Papyrus was actually awake, watching them both just as intently. The older brother jumped as he spoke quietly.

“So Frisk can talk.” he said.

“Y-yeah,” Sans replied, just as quietly “Not very well, by the sounds of it.”

“We should encourage her to talk more.”

“'Her'?”

“Humans have genders, don't they?” Papyrus asked “Frisk looks like a girl to me.”

“Dunno, I guess.” Sans replied “We'll ask in the morning. Get some sleep, okay, bro?”

Sans got up the leave again, Papyrus settling back down. How many hours before he had to get up and go to work? Not enough, that was for sure.

“Sans...”

He stopped. Frisk was still asleep, mumbling incoherently. Sans went to bed.

* * *

 

As expected, Sans didn't get nearly enough sleep. Getting up in the morning was a real struggle, more even than usual, but somehow he managed it. He needed an extra strong cup of coffee, sighing deeply as he shuffled down the stairs. It seemed like Frisk's nocturnal mumblings had inspired some jealousy in Papyrus, as he encouraged the child to say his name as the two had breakfast.

“Pa-py-rus.” he said slowly.

“Paa-pus.” Frisk tried.

“Pa-py-rus.”

“Paa-pap-us.”

“Pa-py-rus.”

“Nyeh heh heh!”

Papyrus burst out laughing, Frisk smiling triumphantly before eating more toast. Even Sans chuckled as he put the kettle on.

“Oh, brother!” Papyrus called as he realised he was there “Check this out! Frisk, say it now!”

“G'mornun.” Frisk parroted loudly.

“Good morning, Frisk.” Sans chuckled.

“Nyeh heh, pretty good, right?!” Papyrus declared “I, the great Papyrus, have taken it upon myself to coach Frisk in elocution!”

“Good work, bro.” Sans congratulated “I can already hear an improvement.”

Both kids grinned happily. Frisk waved their hands excitedly, clearly wanting to speak more now that they had been praised, but finding it difficult to summon the words.

“S-saa's!” they cried “A... um... Alfs?”

“Yeah, we're going to see Alphys again today.” Sans confirmed, not giving Frisk the chance to be disheartened by their mispronunciation “Hope you're ready for more Nu Nu Wacky Tacky.”

Frisk laughed again. Papyrus didn't get it.

* * *

 

While making his own packed lunch, Papyrus made one for Frisk as well (he made one for Sans too, but that was nothing out of the ordinary), which the child was more than happy to accept. The Riverperson was waiting for them to the north of town, once again teasing Sans (who was carrying Frisk on his back) as the two of them stepped on board.

When Sans finally got to work, that old familiar tension greeted him as he walked in the door. Luckily, it seemed to be in its way out, Dr. Gaster throwing on his coat just as Sans removed his, an overnight bag packed at his feet.

“This Kings illness is worse than we thought.” Gaster told him “It may take him a while to recover. They want me to go and tend to him – me! I'm a scientist, what do I know about medicine?”

“More than most monsters.” Sans pointed out.

“Yes, well...” he sighed “I expect everyone to carry on their work as normal in my absence. As for your human – under the circumstances, it's just not a priority right now. You'll have to take care of it a little while longer.”

“Thats...” Sans went to argue, trailing off.

He couldn't really call Frisk a financial burden with them stood right there.

“Oh yes, I forgot.” Dr Gaster managed to pick up anyway, pulling a sizable purse from his coat pocket “The Queen asked me to forward this to you – a little 'thank you' for taking care of the human until they can deal with it.”

Sans nearly flipped his lid, barely keeping his cool as he took his reward – at least now he could afford to feed the kid without cutting into his savings! And since his growth spurt, Papyrus' clothes were getting a little snug. And fridge needed replacing... the more he thought about it, the less money it seemed like, so he stopped, shoving the purse into his pocket. Dr. Gaster adjusted his collar, taking a quick look in the mirror as he headed out, patting Frisk gently on the head.

“Be good.” he bid "Keep Sans out of trouble."

As he shut the door behind him, all tension left the building. The other scientists breathed a sight of relief, almost in unison, and loosened their necktie's a notch. Sans took his off altogether, shoving it unceremoniously in his bresst pocket. He looked over as Frisk tugged on his sleeve.

“Alfs?”

* * *

 

Whatever it was she and Dr Gaster had started working on last night, Alphys was already building it, her desk cleared of plates, books and anime figures to make room for the parts. She was bent over the blueprints adjusting some calculations as Sans and Frisk clambered over the mess on the stairs.

“Hey, Alphys, can I leave Frisk here to watch cartoons again?”

“Hm?”

Evidently not having heard him properly, Alphys looked up.

“Oh, Sans!” she declared “Weren't you going home? The riverperson will be sick of waiting if you don't hurry!”

“That was yesterday, cupcake.”

“Oh... well, then they'll definitely be sick of waiting... Hi, Frisk.”

“Hi!”

“Oh, a whole word today!”

Frisk laughed. Alphys went to get up from her stool, but in her sleep-deprived state, nearly fell straight off.

“Man, I need to go wash my face.” she declared “Oh, yeah, you can leave Frisk in here, it's fine, but there's some new data to process. I think it's on your desk already.”

“Okay, I have just one question.”

“Hm?”

“Why did you give that fridge arms?”

“Mind your business.”

* * *

 

Dr Gaster didn't return that day. Nor the next day, nor the next. Whatever was wrong with the King, it was clearly pretty serious. Sans had met Asgore a couple of times, and he seemed like a genuinely nice guy – he hoped he was going to be okay. With a wife and two kids too... For the sake of the Undergrounds morale, it seemed his illness was being kept a secret, no-one breathing a word of it outside the lab.

As the days ticked on, Frisk opened up more and more, and while never exactly chatty, was soon able to converse in full – if still somewhat stilted – sentences. They also stopped sleeping under the table, bunking with Papyrus most nights, but once or twice Sans woke up to find the kid in his bed. Sans found it amazing, especially considering how abrupt their arrival had been, how easily the child slotted into their lives. Luckily, Frisk had no issue wearing Papyrus' hand-me-down's, and after about a week of gorging like they'd never seen food before, settled into more reasonable eating habits.

After being over half an hour late to work because Frisk REALLY didn't want to get up, it struck Sans as completely unnecessary to drag them along every day. Of course, Frisk was still too young to be left alone, so Sans did the adult thing and enrolled them in the local primary school. Since his school was practically right next to it, that now made it Papyrus' responsibility to get the kid up in the morning (which, of course, had been Sans' nefarious scheme all along).

“So... what exactly are you building now?” Sans enquired, regarding the large cylinder sitting in the middle of Alphys' nest.

“Time machine.” she answered matter-of-factly.

“Why?”

“Well, without Dr Gaster around, research is kinda stagnating.” Alphys pointed out “More than usual, I mean. I figure while he's away, I may as well tinker with some of my own projects.”

“Is that why that fridge of yours now has a wheel?”

“Yes, and mind your business.”

“Why only one?”

“Sans, are you going to stand there asking questions all day, or are you going to build a time machine with me?”

“Your research may be stalled, but that thing Dr Gaster had you build is pumping out more data than I can keep up with.”

“Sounds riveting.” Alphys smarmed “The way I see it, you can die of boredom under a workload you'll never see the end of, or you can stay here with me doing mad science and talking about your brother.”

“Well, when you put it that way.” Sans laughed, hitching up his trouser legs and sitting on the floor amongst the parts, much to Alphys' delight “Pass me that solder. Y'know, the other day, Papyrus...”

* * *

 

It had been some time since W D Gaster had been home. The Kings illness had indeed been serious. Little Prince Asriel had, in floods of desperate tears, admitted to accidentally poisoning him with buttercups, mixing up a recipe that called for 'cups of butter'. While Dr Gaster considered that any recipe that called for entire cups of butter wasn't going to be too good for anyone in the first place, he could see that it was an innocent mistake on the child's part. That other child, though...

The important thing was that the King was recovering. He'd be fine in no time, especially with his wife taking care of him, so the good doctor was finally able to go home. He didn't have any great expectations of what awaited him – he knew everyone would slack off without him around – but he hoped that they had at least achieved _something_ , even if it was only changing that damn coffee filter finally.

It was late when he arrived home, but the door wasn't locked: the last person out must've assumed Alphys would do it, but she always forgot. Gaster took a good look around so he knew exactly what to yell at people for tomorrow; Dr Odin was building a model ship in a bottle on his desk; all the plants were dead from lack of water, AGAIN (he may have to finally give in and get plastic ones instead, but he couldn't stand the way they smelled); that coffee stain on the breakroom floor was still there; and no-one had washed up any cups. Checking the back room past the mirrored hall, he sighed – Doctors Hive and Head were trying to grow narcotics again.

He went to check on Alphys, spying the light in her room still on – the poor love was bent over her desk, screwdriver in hand, fast asleep, her tv playing static beside her. Dr Gaster couldn't help but feel at least a little bit protective of her: she was so young, and such a scatterbrain, she might actually die if he didn't remind her to eat. He tucked her into bed, taking a brief look at the things she'd been building. That robot of hers was coming along nicely, but he couldn't fathom what the thing in the middle of the room was supposed to be.

He looked over the blueprints, noticing Sans' handwriting on them as well. A time machine? He laughed. Very cute. What it was to be young and so full of dreams. He corrected a few mathematical errors before leaving.

Speaking of Sans, he had better have been working on that data. Dr Gaster was pleasantly surprised to find a good pile of it waiting for him, but his smile soon dropped when he read it over – these values were impossible, there was no way Alphys' machine registered these results. Seeing that Sans had clearly made a series of errors to come to these conclusions, Dr Gaster pulled out the raw data to do it over himself.

Dr Gaster, however, was a very smart man, and it didn't take him long to realise that he was coming to the exact same conclusions as his assistant. Thinking it was the raw data that was flawed, Gaster went to check the machine: it was working perfectly, picking up every minute change in the energy levels around it, even tracking him as he got closer and further away.

The data was accurate. The maths was sound. Something was generating a massive amount of energy. Something close enough at hand for the machine to pick it up, making energy like Gaster hadn't seen since he first turned on the core. If whatever it was generated this much energy just be existing, then by Asgore's beard, he may finally have the answer to all their prayers. But what was it?! Where was it?! How could he get it back?!

* * *

 

“Okay, I'm off.” Sans sighed, pulling on his coat “I may be a little late tonight, so remember to lock the door.”

“Okay.” Papyrus agreed, still finishing his cereal “Will do!”

“And you!” he said to Frisk “No more kiss chase! Your constant flirting is making your teachers uncomfortable!”

“'Kay.” they laughed, also still eating breakfast “Buh-bye!”

“Have a good day!” Papyrus finished, giving a little wave as Sans closed the door behind him.

The skeleton didn't expect anything unusual out of the day, so when he arrived at work to find the whole lab in a complete flurry, he was a little taken aback. He twigged immediately that the boss had returned, and his colleagues were trying desperately to hide proof of their weeks of goofing off. What was Gaster going to think about Alphys and Sans' little side project?

“Sans!” Dr Hive called, panicked, when he saw him “I don't know what you did, but Gaster's all fired up! You better get to his office before he explodes!”

Sans didn't hang around, straightening his tie, collar and cuffs as he went. He didn't even get to Gasters office, finding him in that big empty room by Alphys' nest – the doctor had tucked his tie into his breast pocket and rolled up his sleeves, not even bothering to change out of yesterday's clothes as he hurriedly arranged large machine parts.

“D-doctor?” Sans greeted, not used to seeing him to animated.

“Sans!” he almost yelled in return “We need to make the machine bigger!”

“Bigger, sir?”

“Bigger! More accurate! A greater range! Variables, Sans, I need the variables!”

“B-what exactly are we building?”

Alphys appeared, trundling up the stairs from her nest with more parts, shoving a stack of blueprints and notes into his arms.

“A bigger version of this.” she told him “We can start building the base, but we'll need your calculations-”

“As soon as you can!” Dr Gaster interrupted “Yesterday, if possible! Chop, chop!”

“Yes, sir!”

 


	4. The Source

The second Sans walked in the door, he practically collapsed, initiating much fuss and confusion from Frisk and Papyrus. It had been two weeks since Dr Gaster had returned to work, and he had been like a man possessed ever since, hardly sleeping or eating at all as he tried to hunt down the source of the energy spike. All other projects, regardless of what state they were in, were put aside to give priority to this. Alphys had been particularly busy: her new machine was enormous, picking up every energy signature between the start of Waterfall and the Core. Not satisfied with that, Dr Gaster had her create smaller, portable versions, and handed them out to his team, sending them to look under every leaf, rock and snow poff, in every crack in every brick that made up the capital, and everything in between. Sans only escaped this fate because someone had to stay behind and sort through the mountains of data the project was creating. Most of it was worthless, but Gaster wanted every last digit analysed.

“We aren't being thorough enough!” he complained “Something that can create this kind of energy can't just disappear!”

“M-maybe it's in the Core...” Alphys suggested, claws shaking in fatigue and anxiety as she handed them some coffee “Y'know, s-since the machines don't work in there.”

“That'd suggest that it knows that it is.” Sans pointed out as he took his cup “And that it's hiding from us.”

“That's a distinct possibility.” Gaster thought.

He sighed deeply, taking off his glasses and rubbing his temples. He was burnt out: everyone was.

“Are you two _sure_ you didn't see anything?” he asked once again “You would've been close enough to _touch_ it.”

“There was nothing.” Alphys assured for the millionth time “Nothing out of the ordinary, no-one that shouldn't have been there.”

Taking a deep breath, Gaster sighed again, knowing well that the answer would be no different this time, but still... he nodded.

“Okay.” he said, exhaustion radiating from him “Let's all... take the weekend off. No point killing ourselves over this. We'll come back on Monday with fresh eyes.”

He sighed for the final time, drank his coffee in one gulp, and went back to work.

* * *

 

“He hasn't moved.” Alphys reported the next morning “He's been sat there all night, going over the data.”

“You should take him somewhere.” Sans suggested, holding the phone against his head with his shoulder as he made breakfast “The dump or something.”

“Yeah, even I've got cabin fever.” she confessed “But if I take him anywhere he'll want to bring the scanning equipment.”

“I'm sure you'll think of something.”

“Maybe a meal out or something, just to get him out of that chair. Anyway! Thanks for listening to me rant, bonehead.”

“No problem, noodle breath.”

“Have fun with the kids. See you Monday.”

* * *

 

Despite his exhaustion, Sans did have fun. Papyrus, being the responsible kid that he was, made sure the two of them had finished their weekend homework on Friday, so the first order of the day on Saturday was a snowman building competition.

“Papy, that's too tol.” Frisk pointed out.

“What are you talking about?” he protested as he added more snow “It's a perfect representation of me!”

“Ya, 'n five yeas!”

“Don't be jealous! One day, you, too, will grow tall like your big brother!” Papyrus assured “Although, you'll never be as tall as me!”

“Will too!”

“Will not.”

“Will too!”

“Sans, what do you think?”

The kids looked at him. His snowman, which he'd only been half-heartedly constructing, collapsed into a Sans-sized pile of snow. Too tired to put it back together, he sighed.

“Anything's possible.” he answered them.

They looked at him incredulously.

“Hey, don't be so cold.” he replied “This is snow joke, you know. You guys need to start playing n _ice_.”

“Stop.” Papyrus groaned.

Frisk laughed, giving that toothy grin of theirs.

“Papy, why so hot-headed?” the kid added “You should chill out.”

“Sans, you're a bad influence!” Papyrus cried as his brother started to laugh “You should snow better! Nyeh heh!”

He threw a snowball at Sans, hitting him square in the chest.

“Hey, cool it, bro!”

With that, the war of snowballs and bad puns began, the three-way death match lasting well into the afternoon.

On Sunday, Frisk wanted to try building an igloo, which was a learning experience for all of them. Lesson one: don't build a fire inside of an igloo. While the dome lasted long enough for them to have a picnic and tell a few ghost stories (“...and that's how the Blooks saved Temmie village.” “Wowie!”), the heat from their little fire soon melted it through, causing the whole thing to collapse in on them, burying them all under several feet of snow. Luckily, no-one was injured, laughing it off was they dug themselves out.

* * *

 

Monday morning, however, things weren't quite as rosy. Sans woke up to a panicked scream from Papyrus, followed by his bedroom door being flung open and said brother hauling him out of bed.

“Sans, oh my god!” he yelled “Frisk is dying!”

“Frisk is what?!”

Luckily, it wasn't that bad, but to a monster who's never been ill, a bright red face, high fever and difficulty breathing would certainly seem like it.

“Frisk is gonna be fine.” Sans assured Papyrus as he herded him out the door “But the kid can't go to school in this state. You let the teacher know, okay?”

“Okay...” he begrudgingly agreed, casting another worried look up the stairs as he left.

Sans steeled his spine as he picked up the phone, expecting Dr Gaster to tear him a new eye socket for even daring to suggest another day off. Much to his surprise, he sounded more confused than anything else.

“The human?” he repeated “You still have that?”

“Yeah,” Sans confirmed “I guess with the king being ill, everyone forgot.”

“Goodness, it's been practically been 2 months already!” Gaster supposed “I haven't heard you complain once.”

“Eh, the kid grew on me.”

“Inconvenient.” he grumbled “No matter. Get the child ready to go, we'll put them to bed in my room.”

“Seriously?”

“I need you here, Sans.” was the stern reply “There's too much data to sort through to even think about taking any more time off.”

“Frisk is practically the same size as me...” Sans went to protest.

“I'll take care of it.” Dr Gaster interrupted “Just get them ready.”

* * *

 

Getting Frisk up was easier said than done – all the child wanted to do was sleep, which Sans could sympathise with, but it made getting them into their trousers all the more difficult. Sans had just managed to get their shoes on when there was a short, sharp knock on the door, the kind that signalled that whoever was on the other side really didn't want to be. It was none other than Gaster himself, who clearly hadn't taken his own advice to take the weekend off. With no more than a gruff acknowledgement of his assistants existence, he gently, but unceremoniously picked Frisk up, complete with blanket, and walked out into the snow.

* * *

 

Another day of dead ends. All the data Alphys' machine had gathered over the weekend was worthless. Gasters whole being was aching from the frustration and the effort and the sheer crushing disappointment of it all. The Underground seemed awfully small until you had to search every corner of it.

Despite his weariness, Dr Gaster washed his face and put on some fresh clothes. He didn't have much left that was clean, but laundry was very far down on his list of priorities. As he floated silently around his office/ makeshift bedroom (as was his way, his assistants had told him, and it drove them nuts), picking up this and that to put away out of nervous habit, he didn't make a single sound to disturb the human child wrapped in blankets and fast asleep on his bed.

Sans came to check on them every hour – Gaster didn't much appreciate the interruption to their work, but he held his tongue. Sans was a valuable member of his team, so he cut him a little extra slack, especially since he was essentially a single parent. Besides that, Gaster wasn't a sociopath: Frisk, despite being human, was a cute kid, and he didn't wish anything bad on him... her? Gaster willed them to get better quickly so they could go back to school and let Sans concentrate.

He made a mental note to remind Queen Toriel of the child's existence as he left the lab and made his way to the palace.

* * *

 

“Blood pressure is a little higher than normal, but it's nothing to worry about.” Gaster concluded, packing his equipment back into his bag “I suggest you watch your salt intake, but other than that I declare you to be the very picture of health.”

“That's good to know.” Asgore replied as he rolled down his sleeve “I had everybody pretty worried for a while there, didn't I?”

“Yes, well, maybe supervise your children a little closer when they're cooking from now on.”

“Yes, definitely, poor mites. Speaking of 'picture of health'...” the King said carefully as he picked up the cup of tea on the table “Are you doing okay? You look exhausted.”

“Things are pretty busy down at the lab.” Gaster confessed.

“Everything okay with the core?”

“Oh, yes, yes, the core's fine.” he assured “It's something new we're working on.”

“Your barrier project?” Asgore pried.

“Well... yes.” Gaster admitted reluctantly, drumming his fingers on his bag “But it's in its infancy right now. Nothing big to report, I'm afraid.”

“I see.”

Asgore looked disappointed, expression clear on that big fuzzy face of his. It hurt Gaster to see it, making him drum his fingers again. Asgore stood as Toriel bustled into the room, tray of tea and biscuits in her hands, and gave her his usual adoring kiss as she put it on the table.

“Just in time, as always.” he told her “This one's gone cold.”

“You let tea go cold?” Toriel teased her husband “You must still be ill! Back to bed with you!”

They both chuckled. Asgore made himself a fresh cup as Toriel handed Dr Gaster a slice of cake.

“In all seriousness, how was everything?” she asked “All okay, I hope.”

“'The picture of health'.” Asgore quoted “Right?”

“Yes, but watch your salt intake.” Gaster repeated.

“That's good.” Toriel sighed “Putting that aside, how are you doing these days Wing?”

“He's busy with his anti-barrier project.” Asgore answered for him before taking a big bite of cake “They've got some ideas, it seems.”

“He can tell me himself, fluffybuns.” she scolded her husband gently.

Toriel looked at him expectantly, and Gaster felt something unpleasant crawl down his back. There was no way the brains behind the throne would accept 'nothing big to report' and move on. With great reluctance, and a cup of tea handed to him by Asgore, Dr Gaster explained the situation. Toriel listened intently with that serious face she could pull out at a moments notice, nodding occasionally and asking the odd question. Asgore, soon realising it was all a bit over his head, preoccupied himself with his cake and tea.

“So, that's where we're at.” Gaster finished “I've got my team scouring the Underground for the source of the energy spike, but it's proving elusive.”

“I see...”

Toriel put her index fingers to her lips, the way she always did when she was thinking. Gaster hated it when she did that, because it usually meant something bad for him.

“I won't lie.” she said tactfully “I was disappointed when your Core project didn't achieve the desired results. We were still able to make use of your machine to create electricity, so it wasn't a complete loss, but as it's consistently creating enough energy for the whole Underground I can't really see the need to create more.”

“If I'm correct, whatever this source is might generate enough energy to break the barrier once and for all.” Gaster insisted.

“You said that about the Core.” Toriel reminded him.

He felt his stomach churn. She was right, which only made it all the worse. His Core, for all the good it was doing the Underground now, had been a complete failure in it's intended design. This time, however, he wasn't going to fail.

“I'm sure about this one, Toriel.” he said seriously, looking her right in the eye “If we can find the source-”

“I'm not interested in 'if'.” she interrupted “'If' we can find it, 'if' we can replicate it. 'If' isn't going to help the people. 'If' isn't going to help raise morale.”

Her eyes narrowed unhappily. Gaster knew that she hated being harsh, but as the queen it was her duty. Asgore looked between the two of them cautiously. Toriel sighed, finished her tea, and stood.

“I don't want you spreading 'if's.” she ordered “False hope is worse than no hope, as it only leads to disappointment. When you have some real, tangible results, then I'll listen to your ideas. Take care of yourself, Wing.”

With no more ceremony, she left. Asgore fingered the rim of his teacup awkwardly as Gaster let her words sink in.

“She doesn't mean-” Asgore started.

“I know what she means.” he assured.

“It's just... you know how important your work is to everyone. Our freedom is pretty much reliant on your research... But, y'know... no pressure...”

He sipped his tea self-consciously. Gaster couldn't even respond, feeling the weight of the royals words on his shoulders. Asgore startled when he suddenly stood, finishing his tea and picking up his bag.

“Thank you for the tea, your highness.” he bid “I'll be back when I have some real results.”

“Wing...”

He didn't hang around to hear what he had to say, marching straight out without so much as a goodbye to the footmen. He had work to do.

* * *

 

The lights were all still on, despite the late hour. Dr Gaster expected to find the place abandoned, his staff having taken off as soon as possible on the one day he wasn't around. To his great surprise, he found all but two of them in the break room, sleeping on chairs and sofas and under tables, curled up in piles with bandages and plasters covering their poor, abused feet. Dr Odin had kicked off his shoes as he sprawled over the couch, Dr Head snoring loudly. On the coffee table lay a pile of the portable scanners, each with a note attached in Alphys' handwriting, labelling them as having been processed and ready for use in the morning.

They were all trying their best. The weight got heavier. Gaster proceeded to Alphys' nest, where she too was asleep, bent over her desk as her computer compiled the data from the last scanner. She still had a half-empty cup of noodles in one hand, fork in her mouth. Gaster removed it, covering her with a blanket before unplugging the scanner to take back upstairs. He would have to wait until the morning for the data.

Lastly, he went to the back office to check on Sans: sure enough, there he was, asleep in his chair with take-out boxes littering his desk. If he was still here, then Frisk would be too, so Gaster went back to his office/ makeshift bedroom. He was slightly surprised to find Papyrus there as well, sleeping right next to the human child with a book of children's stories in his hands. Gaster immediately felt guilty: if Papyrus was here, it meant Sans was taking this project seriously, staying overnight... The weight wore him down. He needed rest, but he had to work. There was so much to do...

Putting the scanner on the dressing table, he kicked off his shoes and pulled off his tie, whole body aching to remind him he hadn't had any real sleep in weeks. He collapsed onto the old futon he had used before he bought he bed, back when his back was stronger, feeling whatever energy he had left draining from him. His mind was all abuzz, refusing to turn off. It ran through numbers and variables and possibilities and... 'ifs'... it just kept buzzing. He just wanted to sleep. He just wanted to work.

He looked at the scanner on the table, where it sat like a useless paperweight. He picked it up, fiddling with it absently. Alphys was very resourceful: the body of the scanner was made of an old hand-held vacuum cleaner, the screen an old gaming device and the gauge a speedometer from an old car. He was impressed, wondering what she could do if given some actual resources to work with. Still absently fiddling with it, he flicked the switch to turn it on.

It immediately screamed at him, the sound startling him to much that he nearly dropped it as he fumbled to turn it off. He looked up: somehow, it hadn't woken the children. Turning the volume right down, he flicked it on again. The gauge went nuts, almost breaking from how quickly it hit the top, the screen flashing all sorts of crazy numbers.

Gaster went cold, everything around him going still. The energy spike... it was _here_! Silently, tentatively, as if sudden movement might scare it off, Gaster stood, holding the scanner out in front of him. He moved it right: the readings dropped slightly. He moved it left: same result. The readings were coming from the centre. The readings were coming from the bed.

He held the scanner over Papyrus. The readings dropped. He held it over Frisk. The readings hit maximum. Ever the professional, he checked the other side of the wall, but again the readings dropped. He got another scanner, just to make sure, but the result was the same. Gaster took a moment. He went back to the futon, sat down, and laced his fingers thoughtfully, pressing his index fingers to his lips.

Well, now...

 

 


	5. The Doctors Intentions

**Notes for the Chapter:**

> Hey! Thanks to everyone who's been leaving 'kudos' on my work - I really appreciate it!

Gaster was bizarrely calm. He let everyone sleep in, still lay where they passed out, and upon awakening, simply sent them home for some proper rest. When asked about the scanning, he simply told them not to worry about it, but refused to say more. He even gave Papyrus a little money so he could buy himself some lunch before sending him off to school, refusing Sans' efforts to pay him back. Frisk was still unwell, but Sans managed to get the child up long enough to get some porridge and water into them before letting them go back to sleep. Since Sans couldn't exactly leave them here and go home, he joined Alphys down in her nest for an exhausting morning of doing absolutely nothing.

Oddly, the first thing Dr Gaster did was clean. He cleared away all the clutter that the team had let pile up over the last few weeks, putting things neatly away in cupboards and drawers or in the bin, washing up all the plates, doing all the laundry, vacuuming, dusting, polishing... Sans and Alphys only became aware of his activities when he unceremoniously threw them out of the nest to clean there as well.

“Something's definitely bothering him.” Alphys knew.

“Maybe the king and queen said something?” Sans thought.

“Maybe... wow, I forgot how big this place was when it was clean.”

“He even got that stain off the ceiling.”

“Holy cow!”

It was only after lunch (where the two of them tried disastrously to make chicken noodle soup for Frisk), that they were allowed back downstairs. To Alphys' great relief, Gaster hadn't thrown anything away, but had really gone to town organising all the disparate cogs, wires and other assorted parts.

“Oh my god, I can see the floor.” Sans said.

“Was my bed always that big?” Alphys asked “No-one's done my laundry since I moved out of my parents house.”

“How are your parents these days?”

“They're good. They keep asking me when you're coming over again. They also mentioned something about grandchildren... I think they think we're a couple.”

“A couple of what?” they both managed to say at once, making them laugh “Jinx!”

* * *

 

It was late afternoon when Dr Gaster finished cleaning, and the lab looked absolutely resplendent. Freshly showered and dressed in clean, ironed clothes, Gaster himself wasn't far behind.

“Looking spiffy, boss.” Sans noted when he went to take Frisks temperature “You got a date or something?”

“Thank you, Sans.” Gaster replied “But it's nothing like that. We're embarking on a new stage in our research, and we need to approach it with the right frame of mind. You know what they say: messy home, messy head.”

“You're literally the first person I've ever heard say that.” he teased.

Sans looked at the thermometer – higher than he would like, but nothing to worry about (at least according to the literature). He became self-conscious when he felt Dr Gaster looming over his shoulder.

“Something on my head?” he asked.

“What? Oh, no...” Gaster chuckled “I was just thinking... curious... say, would you consent to us taking from readings from Frisk?”

“Excuse me?”

“You know, recording brainwaves and vitals and such.” he clarified “Nothing intrusive. I mean... we've never examined humans using our modern technology. I'm curious as to how the results would differ from us monsters. Aren't you?”

“Well, when you put it that way...” Sans conceded, the scientist in him admittedly curious “But nothing invasive, right? No needles?”

“Nothing like that.” Gaster promised again.

“Well... as long as the kid doesn't mind...”

“Wonderful!”

Gently enough not to wake the child, Dr Gaster lifted them out of the bed and proceeded down the hall. Sans followed him to the observation room beside the lift, where a bed and monitoring equipment had already been set up.

“You were pretty confident I'd say yes.” Sans chuckled.

“Well, you're a scientist, same as me.” he replied as he tucked Frisk in and placed the sensors on them “You know, you should go home tonight. Don't make Papyrus trek all the way out here again so he doesn't have to spent the night alone.”

“But Frisk-”

“Alphys and I can take care of Frisk. They'll be asleep most of the night anyway, and besides, you're only on the other end of the phone.”

“I guess...”

“And I'm certain.” Gaster assured “Go home. Sleep in your own bed. Everything will be fine.”

* * *

 

Vital signs normal. Brainwaves normal. Magic levels normal. Physically speaking, Frisk was an absolutely normal child. Their previously broken leg was still a little tender, flinching when prodded, and they had some years-old scars which gave Gaster pause to wonder what kind of life the child had been living before falling into the underground, but besides that there was nothing at all exceptional about them.

Of course, Dr Gaster didn't discount the probability of soul power: when Alphys was asleep, he hooked Frisk up to the large machine the two of them had built, intent on analysing every last unit of data it produced, but was immediately overwhelmed by it. He laced his fingers, resting his elbows up on the dash as the numbers scrolled by faster than any conscious mind could read them.

Human souls... ancient legend supposed that it would take the combined power of every single monster soul to equal that of one human one, and now that Gaster had the numbers in front of him, he could see that was more than just idle speculation. The power of Frisk's soul... it was incredible. He had to know more.

* * *

 

Frisk was feeling much netter in the morning – still not quite school-worthy, but well enough to be sat up in bed eating cereal with Alphys when Sans arrived for work. Gaster, however, was nowhere to be found.

“He's gone to the barrier.” Alphys explained, mouth still full of food “Didn't say why. Took scanners with him, though.”

“Did he want us to keep scanning?” Dr Head wandered.

“I'm scared of what he'll do if we make a mess after he went and cleaned everything.” Dr Hive agreed.

“I'm going to use the opportunity to clean up my desk.” Dr Odin announced, adjusting his glasses uncomfortably “Then if Gaster gets mad, I can just say I was following his example.”

There was a general murmur of agreement before the scientists dispersed to their desks. Since Sans and Alphys had had the treatment yesterday, they found their plans more open.

“Sooo,” Alphys suggested “Mad science?”

* * *

 

Gaster returned later in the afternoon, looking, quite frankly, like he had gotten laid: he was clearly tired, noticeably relaxed, and had a grin of self-satisfaction spread across his maw like he had figured out all the secrets in the universe. The second he walked into the lab, he knew he was the centre of the attention of hos staff, dropping his equipment on the nearest table the way a rock-star drops a mic.

“Seven.” he announced “It takes the power of seven human souls to destroy the barrier.”

The room instantly erupted into questions, but Gaster held up his hand, silencing them all.

“For a single person to simply pass through the barrier, it takes the combined strength of a human and a monster soul.” he went on “I don't have to tell my esteemed colleagues that seven is a much smaller number than several thousand. You know what this means.”

His grin spread from ear to ear, almost manic in ecstasy, with hints of malice on its edges.

“We can destroy the barrier.”

“But where are we going to get seven human souls?” Dr Odin pointed out “In thousands of years, only a handful of humans have fallen down here!”

Without missing a beat, Gaster turned to Frisk, who had been playing snap with Dr Hive prior to his arrival, and with two steps of his long legs was beside them. Sans felt something unpleasant crawling up his back as Gaster smiled.

“You'll help us, won't you, Frisk?” he asked them “With your help, we can all be free. We need to study your soul, understand what it's made of, how it works. If we do that, we can make new ones, artificial ones, ones powerful enough to free us all. You'll help us, won't you, Frisk?”

Frisk was obviously overwhelmed. They looked all around for an adult to give them some clue how to respond, but even Sans was silenced by the pressure in the room. With all eyes on them and the future of the entire underground on the line, the child could only nod, mute and unsure. Gaster broke out into another grin, one that threatened to break his face completely.

“Thank you, you marvellous child!” he cried, standing and spinning theatrically on his heels “The future of monster-kind has never looked brighter! Everyone, to work!”

* * *

 

“You want... what?”

Gaster dabbed his lips with the fancy napkin he had been provided with, putting his knife and fork side-by-side on the bone china plate. The children had finished their dinner long ago and were now playing in their room, Asgore doing the dishes while also trying to listen to what Gaster had to say. Toriel, having an inexplicable hatred of leftovers, immediately dished him up a plate of what the family had had for dinner and refused to let him discuss work until he had eaten at least half of it.

“It doesn't have to be blood.” he assured “It can be hair, saliva... anything from Chara's body will do the trick.”

“And... what do you want it for?” Toriel asked, having picked up on his evasive tone.

“Just a little side project my team is working on, looking at genomes and chromosomes and such. We've got all sorts of monster volunteers – skeletons, spiders, slimes, dogs, lizards – but it would be really interesting to have a human sample as well. For comparison.”

“Huh...”

Toriel sat back in her chair and had a little think. It was a simple enough request with a simple enough explanation, so there was really no reason for her to refuse.

“I suppose... when Chara grows up, it would be nice to know if the child could start a family down here...” she pondered.

“It would be easy for us to find that out.” Gaster assured “But it's a little early to be thinking about grandchildren, isn't it?”

“Humans grow up very quickly, it's best to be prepared.” Toriel said “I suppose it won't do any harm, but please don't make a habit of including Chara in your research.”

“I'm sure this will be the last time.” he promised.

“We cut Chara's hair the other day.” Asgore remembered, putting the last of the dishes away in the cupboard “I think it's still in the bathroom bin, if that'll do.”

“That would be perfect.” Gaster agreed.

Toriel got up and disappeared down the hall, returning a few minutes later with a well-stuffed envelope. Gaster took it graciously and tucked it inside his breast pocket.

“It's probably best not to tell the child about this.” he suggested “You know how children can be, letting their imaginations run wild. Especially with you thinking of grandchildren already, Toriel.”

“I'm glad we could help you with your research.” Asgore chuckled “Next time, just come by for dinner, okay?”

Gaster kept that amiable smile of his as he said his goodbyes, proceeding swiftly from the palace grounds and down the path to the long lift that lead to hotland. Dr Odin stood there waiting, leant against the wall with his arms crossed, looking confused and agitated. Seeing Gaster approaching, he stood straight, taking the envelope that was handed to him. Seeing what was inside only left him more confused.

“What's this for?” he asked.

“Plan B.” Dr Gaster admitted, pressing the button for the lift “If our research with Frisk ends in a dead end, I want to have a back-up.”

“A back-up?” Dr Odin clarified “I'm... I'm not sure I understand what you mean?”

“Dr Odin, my _friend,_ ” Gaster began as the door to the lift opened “I have a special project for you.”

 


	6. The Real Purpose of the Tests

**Notes for the Chapter:**

> I've got a couple of comments now, as well as a handful of kudos! I'm glad someone out there is appreciating my work :) Please enjoy this next chapter!

Sans was decidedly unhappy. 'Study' the kids soul? What did that entail? Not to mention the fact he felt like Dr Gaster had lied to him – out of 'curiosity' he had hooked Frisk up to the monitors, and he just happened to find out all this useful information that was vital for their research? Sure, coincidences happen, but that was a little too neat, especially for someone as meticulous as Gaster. He wished he had said something at the time, not been so cowed by peer pressure, stood up for the kid... of course Frisk was going to say yes, with all the adults looking at them like that.

Dr Gaster wanted everything ready to go, clearing the lab of all distractions. While he allowed Alphys to keep her weird fridge, he insisted on getting rid of the time machine, giving her three days to move it before it was scrapped. Never able to throw anything away, she all but begged Sans to store it in his utility room, taking it apart piece by piece. Seeing how obviously unhappy her friend was as the two of them put it back together, Alphys couldn't help but be concerned.

“Penny for your thoughts, bonehead?” she asked, putting her spanner down on the floor.

Sans sighed, grinding his teeth. He went to speak, but nothing came out, so he crossed his arms, quickly uncrossing them again and scratching his head.

“I'm not happy with this.” he confessed “Not at all. How exactly is Gaster going to study Frisks soul? And if he finds what he wants, what then? Artificial souls? How do we know that's even possible? No magic or technology has ever been able to make one. And what will happen to Frisks soul after being 'studied'? Last I checked, the kid is still using it!”

Alphys nodded as she listened, taking in all his concerns. She let him rant for a while before formulating an answer.

“I hear what you're saying.” she assured “But if anyone can make an artificial soul, it's Gaster. We've never had technology as advanced as we have now, so who knows what we'll find out! As for Frisk...”

Alphys wiped the oil from her hands, tucking the rag into her belt before placing a reassuring hand on Sans' shoulder.

“You'll be there. And I'll be there. And do you honestly think Dr Gaster will let anything bad happen? He won't put Frisk's life in danger, not for all the research in the underground!”

Sans wasn't convinced. Not after the way he saw Gaster smile.

“I don't like it.” he repeated.

“Well, unless Frisk actually says something, there's nothing either of us can do about it.” Alphys pointed out “Try to have more faith in Dr Gaster, he's not the tyrant everyone makes him out to be.”

* * *

 

Gaster designed the tests, Alphys building machines to his specifications, while the other doctors assisted, analysing the data and keeping Frisk fed and occupied between experiments, even fitting in maths and science lessons when they could. Despite his misgivings, Sans found himself falling in line with them pretty quickly. Their enthusiasm was infectious, and the fact that they were finally getting some positive results... he soon forgot his concerns as the data came piling in.

Frisk couldn't be prouder: seeing the reactions the scientists had every time they got the results they wanted, the child tried harder and harder to please them, looking genuinely devastated when a test failed. Sans would pat them on the head and tell them that they were doing just fine and not to worry, but they would always try harder next time.

The tests were surprisingly innocuous – puzzles, riddles, logical conundrums – the kind of stuff Frisk did with Papyrus anyway. After Dr Gaster accidentally spilled some hot coffee on the child's hand after a particular test, they found that physical stress increased the result significantly, so along came the jumping jacks, sit-ups and running on the spot. The whole thing was incredibly tiring, but the child refused to give up.

Aside from Alphys, who was always busy building Gasters latest idea, there was one doctor who was conspicuous in his absence: Dr Odin. Dr Odin was a gangly sort of fellow, with a wide mouth, thick round glasses and weirdly long arms: he stood out amongst the staff as one of the tallest. H would often joke that his nickname was 'can you get that down for me?' because of how many times he was needed to reach things on high shelves, so the fact that the biologist wasn't around for any of their tests was soon noted by the other staff.

“Dr Odin is working on something else.” Gaster dismissed “We don't particularly need his expertise at the moment, but you know how he hates being a loose end. I gave him an interesting side project to keep him occupied.”

“But where is he?” Dr Hive pried “I see him in the lab a couple times a day, but he's never at his desk.”

“In the field, probably.” he replied “He's studying the echo flowers.”

Dr Odin hated field work – no wonder he looked so dour and stressed lately.

* * *

 

After a few weeks of testing, Gaster had another rock-star announcement, pulling everyone (excluding Dr Odin) into the main lab.

“We found it.” he told the staff, hands raised like a preacher in sermon “That 'special something', that unique trait that gives human souls so much strength. It's identifiable, quantifiable and, with a little more research, replicable.”

There was an excited murmur in the crowd. Gaster waved his hand to hush them, turning his eyes to Frisk.

“The element was at its strongest when our subject was put under stress, but refused to give up. For that reason, at least for the moment, I'm dubbing the element 'determination'.”

Another whisper of excitement rippled through the scientists. Gaster clasped his hands together, all smiles, like a proud teacher looking upon a graduating class.

“In celebration of our discovery, I'm letting everyone go home early today.” he announced “So let's all come back tomorrow 'determined' to do our best for the underground. Alphys dear, it's going to be a late one for us – now we know what we're dealing with, we're going to need something special to contain it.”

“Alright!” she agreed with a theatrical fist pump “I'm 'determined' to get started!”

“That's the spirit!” Gaster enthused “Come now, let me order us a pizza and we'll discuss our plans.”

Gaster glided through the crowd as it dispersed, putting his hand on Alphys' shoulder as he guided her out of the room.

“See you two tomorrow!” she bid over her shoulder.

“C'mon, kid.” Sans said, ruffling Frisk's hair playfully “Let's get outta here. We'll go by the store on the way home, you can pick out anything you like.”

“Bunnies!” Frisk knew immediately.

“Sure.” he chuckled back “A whole bakers dozen, why not?”

Frisk flashed that toothy grin of theirs and ran off to get their coats. Sans stuck his hands in his pockets and ambled after them, nearly jumping clear out of his shoes when a shadow sudden shuffled in the corner of the room.

“Odin!” Sans exclaimed like a curse, clutching his chest “Don't do that, you nearly scared the soul outta me! Where did you pop out from anyway?”

“Hm? Oh, hello Sans.” Dr Odin greeted wearily “It's awfully quiet up here.”

“Good god, man, you look like death.” he noted “You feeling okay there?”

“Yes, my research is just at a crucial point at the moment.” Odin replied, pushing his glasses up his face “Haven't gone home in three days. I'm tired.”

“What was it you were studying again?”

“Echo flowers.” he answered abruptly.

“Right...”

Frisk reappeared, ready to go, and handed Sans his coat. Preoccupied with putting it on, Sans didn't notice Dr Odin wince at the sight of the child, averting his gaze.

“Y'know, Gaster sent everyone home early.” Sans mentioned “You should go home, have a nap, see your kids. How old are they now?”

“Seven and five.” Odin confessed, voice distant.

“If you haven't gone home in thee days, then you definitely should now, if only to give your wife a break!”

“Yeah...”

“Buh-bye Mr Odin.” Frisk bid as Sans took their hand and headed for the door.

“... bye, Frisk...”

* * *

 

“Wow, kid, how do your clothes smell so bad?” Sans teased as he threw them in the washing machine “You keeping a skunk up your sleeve or what? C'mon, shirt too.”

Frisk peeled off the pink and blue hand-me-down shirt and gave it to Sans, pausing only to also strip off their socks before heading to the waiting bath.

“Hang on!” Sans called sternly, making the child flinch.

Sans wasn't the type to raise his voice, so Frisk immediately thought they had done something wrong, shrinking away instinctually as he walked forward and grabbed their arm.

“This bruise is new.” he noted “And this one. Did you get these during the tests?”

Frisk noticeably relaxed, nodding.

“Mm.” the child confirmed “Reflex test.”

“They shouldn't be hitting you that hard for a reflex test.” Sans grumbled “And what's this cut here?”

Frisk just giggled, throwing Sans off. The child kissed him on the cheek before running off to the bath.

Frisk may not have been bothered by the cuts and bruises, but Sans was. The child never spoke of their life above ground – the silence in itself spoke volumes – but the scars it left on their skin gave Sans cause for concern. The idea of purposefully hurting a child, let alone bad enough to leave a permanent mark... well, there was a reason he was the one raising his brother. And now that he was Frisks guardian as well, he had sworn not to let any more harm come to them. Realising he was just stood there, staring at the machine, the skeleton got on with the washing.

* * *

 

There was still a little time before Papyrus was due home, so Frisk and Sans sat down on the sofa watching television, the elder towelling the youngers hair dry as they sat between his legs. Those bruises still bothered him.

“Hey, kid?”

“Hm?”

“Y'know, this whole 'soul experiment' thing,” Sans began “You can stop any time you like. I'm not gonna force you. You wanna go back to school, you just say so. If it hurts or it gets too scary, just say the word and I'll pull the plug.”

“What about your job?”

“There'll always be other jobs, but there's only one Frisk.” he reasoned.

The child smiled happily at this statement, leaning back against the skeleton as they watched TV.

“S'okay.” Frisk assured “Doesn't really hurt, an' I don't mind being sleepy. I like the lab – everyone's nice to me, and I like it when they're happy. I wanna help everyone.”

“You wanna go back up to the surface?”

“No.” Frisk replied immediately “There's nothing up there.”

“Nothing?” Sans pried.

When no more information was forthcoming, Sans put his arms comfortingly around the child's shoulders, feeling them sigh.

“I was alone.” Frisk reluctantly admitted “I had no home. I was always cold... I like it better here. Even when the barrier is gone, I want to stay. Or if it never goes, that's fine too.”

“And if we all went to the surface, would you still stay here?”

“You wouldn't leave me!?” Frisk cried.

“Of course not.” Sans chuckled “I'm just pulling your leg. How long have you been stuck here with us now? Four months? Human or not, you're a skeleton now kid, whether you like it or not.”

“Frisk the skelly.” they laughed “I like it.”

“Yeah, me too.”

* * *

 

“Alphys really is one hell of an engineer.” Gaster enthused, admiring one of the finished containment vials “We've already made the seven we need, plus a couple extras just in case.”

“She always was your favourite.” Dr Odin mentioned, face lit only by the harsh glow of the computer screen.

Dr Gaster put the vial carefully into the machine before floating across the darkened lab and putting his his hands on Odin's shoulders.

“My favourite, perhaps, but not my best.” he assured “How is your research coming along?”

“Not much has changed since yesterday.” Odin sighed “They're slightly bigger, but still not viable. I've accelerated their growth, and while they're starting to look more... human... they don't show any indication of having souls.”

“Perhaps they need a catalyst.” Gaster supposed, regarding the small forms, suspended in the faintly glowing tubes all around “A little something extra from their 'mother'... I'll think of something. You keep at it.”

He patted Odin encouragingly before leaving. It was so cold and dim down here, Gaster could hardly stand it, feeling the temperature raise pleasantly as he ascended the stairs, but it was necessary for the equipment. He got to the top of the stairs, closing the door securely behind him, and stepped into the decontamination chamber, which sprayed him down in its cold fog (he hated that smell...). When the door released, he stepped out into the room, taking a quick look back. No light could be seen in the cracks: no matter how you looked at it, it looked like a normal fridge. He closed the door just as Alphys bustled in, looking over the latest blueprints with a concerned expression.

“Oh!” she declared up on seeing him “There you are! I was looking all over for you... where were you?”

“Just adjusting some wiring in the back room.” he lied “You wanted me?”

“Yes, this latest machine,” she confessed, holding the blueprints out for him to take as she walked forward “I-I just wanted to confirm... I mean... I'm not saying you made _errors_ , but...”

“You've seen a mistake?” Gaster asked as he took it, looking it over.

“Well... I just... It seems like... l-like this m-machine would cause Frisk, well... _pain_ , and... well, q-quite a bit actually...”

He looked up at Alphys, who flustered uncomfortably, not able to look at him. She was cute when she flustered.

“I-I know you wouldn't want to hurt Frisk on p-purpose, so...”

Gaster smiled, putting his hand on her shoulder in a paternal fashion.

“My goodness, you're right!” he declared “I must have been half asleep when I put this together: thank you so much for pointing it out to me. This could've ended quite badly!”

Alphys finally looked at him, face a picture of relief, and smiled.

“Of course!” she said “I mean, everyone makes mistakes when they're tired, right?”

“I'm glad at least one of us was paying attention.” Gaster complimented, gently turning her around and herding her out of the room “You just finish the DT extractor, and I'll fix these plans. Don't forget to get some sleep tonight as well.”

“Same to you, Dr Gaster!” Alphys teased “I'll be in the extractor chamber if you need me.”

“Of course. Good night, Alphys dear.”

Watching her leave, Dr Gaster folded the blueprint up and slid it neatly into his breast pocket. Of course sweet little Alphys wouldn't be okay with causing their subject pain, even if their research had shown that that was when their 'determination' was at it's highest. Turning on his heels, he opened to the door to the next fridge. He would just have to build the machine himself.

 


	7. How Far He'll Go.

“Hey, Sans?”

“Yeah, bro?”

“When will Frisk come back to school?”

Sans looked around, hands still submerged in the soapy water in the sink, at Papyrus, who stood awkwardly in the kitchen doorway, scratching his arm and looking away to the side.

“I know that... you must have a good reason for not letting them go.” he went on “But... everyone misses them. I miss them. Frisk is always so tired when you get home, we don't even play any more. I know I shouldn't ask what you're doing at the lab, but... I guess I'm just worried...”

Sans heart-strings pulled so hard, they threatened to pull it right out of his chest. He dried his hands on a tea towel as he thought of a response.

“Soon.” he assured “Our research is nearly finished. I'm sorry you're feeling lonely, bud, but it's not forever. Things will go back to normal soon.”

“Promise?” Papyrus insisted.

“You know I hate making promises.” Sans sighed “But I'm gonna do my best. Just hang on a little longer, okay?”

“Alright...” he agreed reluctantly “I believe in you, Sans! I'll just have to be an even better big brother to Frisk for a while! I can do it!”

With a shuffle of his feet, Papyrus left, but Sans could tell that he wasn't entirely convinced. 'Big brother'... Papyrus had called himself that before. It was hard to keep things from Papyrus, especially as he and Frisk still shared a bedroom, and the skelly was surprisingly observant. Sans had more than just his and Frisk's feelings to consider.

Still stood there in the kitchen, Sans had himself a moment of realisation: he had formed a family. Despite telling Frisk several times that the child was one of them now, and making all the massive, life-altering changes he had needed to to make a home for Papyrus, the weight of it had somehow snuck up on him. He still remembered vividly the life of a care-free college student, a bachelor young lab-tech making it on his own in the world. When the shit hit the fan those few years ago, Sans had tipped his whole life upside down for his brother, and it had been a major disruption: being a parent was hard, and he didn't have the slightest idea what he was doing for the longest time.

When did it become this easy? Now that there was two of them, shouldn't it be even harder? He used to lay awake at night, wondering if he was doing the right thing, setting the right example, providing enough. When had he stopped doing that? When had his most anticipated day not been payday, but the days he had planned to do something with those two? When had Friday not been celebrated at his excuse to get drunk and party, but to spend quality time with Papyrus and Frisk? It had all snuck up on him, like a shadow in the night. Time moved far too quickly.

Finally finishing the dishes, Sans locked up the house and went upstairs, poking his head into the kids room. Frisk was obviously exhausted, sat in Papyrus' lap as he read them a bedtime story. Sans stayed to listen, leaning against the door frame. He was happy. This family was an unusual one, patched together from cast-offs and held up with nothing but love and determination, but he was happy all the same. The kids were doing their best, it was time he started doing his as well. As he bid the kids goodnight, going to his own room to sleep, he came to a decision.

* * *

 

“You... what?”

Gaster looked at Sans like he had grown a second head. Despite the early hour, Gaster was already eye-deep in readings and other paperwork as he sat at his desk, once again looking like he hadn't gotten any sleep.

“I know it's not what you want to hear.” Sans went on “But I'm Frisk's guardian, and I have to put my foot down. They've missed too much school already, and all these tests are having a negative impact on them.”

“They've not said anything.” Gaster pointed out.

“Exactly.” Sans challenged “The kid almost never talks these days, so I know they're upset. I'm sorry, boss, but I'm not taking no for an answer on this, even if it means my job.”

Dr Gaster took a moment to respond. He looked at Sans carefully, thoughtfully tapping on his full cup of coffee. He only looked away to take a long sip, putting the mug down and turning to Sans properly, poising his long fingers against each other as he found his words.

“I hear what you're saying.” he said “Of course, it's your responsibility to look out for the subject's well being – I would expect no less of you – and they have been off school for a while... I'll tell you what, Sans: give us two more weeks. In two weeks, regardless of what state we're in, we'll stop experimenting on Frisk. They can go back to school, and we'll concentrate on our artificial soul project. What do you say?”

Sans was unhappy with the compromise, but in truth he hadn't expected Gaster to give in without a fight. They must clearly be further along in their research than he thought if Gaster was willing to let Frisk leave. Reluctantly, he held out his hand.

“Two weeks.” he agreed “After that, Frisk never comes back to this lab.”

“I won't lay a single finger on them.” Gaster agreed, shaking the offered hand.

* * *

 

“Now that we've identified 'determination', our focus is going to be shifting,” Dr Gaster announced to the gathered scientists at the morning meeting “We're moving away from analysis and onto extraction. The subject's... Frisk's well being and safety are of the utmost priority during this stage, so things are going to be organised a little differently now. Alphys dear...”

Alphys, who had been concentrating on not spilling her coffee as she drank it, paid attention as her name was called.

“I'm going to need you to stay with the DT extractor while it's in use, just to make sure nothing goes wrong.” Gaster informed her “We can't afford to waste even a single drop.”

“I'll watch it like it's a Mew Mew special edition OVA!” she swore.

“I knew you would.” he beamed happily “The rest of us will be in the extraction chamber with the subject. Dr Hive, I'm counting on you to monitor the machine from the our side – any fluctuations, any errors, I'll be counting on you to notice and shut the process down. I'll give you a run-down on what you need to do.”

“It's because I don't have eyelids, isn't it?” Dr Hive joked.

“Yes.” Gaster replied plainly “Dr Head, your job is going to be running the extractor.”

“I hope the buttons aren't too small.” he fretted.

“I built them with you in mind.” the boss assured “Sans, I'm putting you in charge of Frisk – if they seem like they're uncomfortable or in pain, I'm relying on you to let us know.”

“... right...”

“And Frisk,” Gaster started, kneeling down so he was face-to-face with the child “This is the very last thing we need you to do. You've done so well, you've done more for the sake of monsters than any other human in history. Thanks to you, we've never been closer to our freedom. Can we count on you for this final stretch?”

Frisk nodded, but still covertly took Sans hand. Gaster pulled that smile that Sans was starting to hate, taking the child by the shoulders and herding them gently away from Sans.

“We don't want to spoil your clothes, so if you go with Dr Hive, he'll get you changed and show you the extractor.” he explained “Go on – Sans will be along in a minute.”

Frisk gave Sans a look, but did as they were told, going with the other doctors as they dispersed further into the lab. Picking up that Dr Gaster had something to say to him, Sans waited, readying himself for being fired. Instead, Gaster got up from the floor, going over to the table and picking up a mug, which Sans recognised instantly as his, and gave it a stir with the spoon.

“I just wanted to say, Sans,” Gaster began “About earlier. I'm not angry.”

Turning back around, he walked back and gave Sans the mug. The hot coffee inside was exactly the way Sans liked it – he could even smell the shot of hazelnut syrup that he swore no-one knew he always had in it.

“It must be difficult, being a parent.” he went on “I'm sure such long hours here haven't been helping, with poor Papyrus home alone as well. I'm happy to have someone as responsible as you on our team, but I want you to know – you don't have to do this all alone, you know? I like to think of you all as family, and family looks out for each other. It's so easy for us scientists to lose sight of our hearts in pursuit of knowledge: this morning, you reminded me of the importance of letting Frisk be a child. You're right of course, such time is so fleeting. So I wanted to take this time to remind you – you're an important part of this family. No matter what decision you make, we'll all support you.”

Sans was taken aback – it wasn't like Gaster to be so... gushing.

“Oh... thank you.” he responded, tone betraying his confusion.

“Now, drink up your coffee.” the doctor insisted, giving the bottom of the mug a tap up “You'll need your energy for the day ahead.”

With a gentle smile, Gaster went to follow the other scientists. Sans didn't want the coffee to cool, so drank it in practically one gulp, throwing the mug in the sink before following. While still a little thrown by Gasters words, it was a weight off to know he wasn't going to have to find another job. Despite everything, he really did like being a physicist and working with his colleagues every day. For the first time in a long time, it seemed like everything was going right for him.

That DT extraction machine sure was freaky looking. Was Alphys going to be okay looking at that thing all day?

“Hey, bonehead!” she called, appearing from behind the machine and beckoning him over.

He did as asked, helping her down from the suspended apparatus and onto the safe floor.

“Is everything okay?” she asked “I couldn't help but sense some tension earlier.”

“Yeah, it's fine.” Sans assured her “I just put my foot down about Frisk going back to school. I thought my neck was on the chopping block for sure, but it all worked out.”

“Geez, did you tell your face that?” Alphys teased, taking a rag out of her pocket and wiping the top of his skull with it “You're caked in sweat, here!”

“Huh?”

Sans reached up and touched his forehead: sure enough, it was moist.

“That's weird... and new...”

“Sans?!”

Alphys caught Sans by the shoulders as he fell, bending over double as he threw up the entire contents of his stomach. She shrieked a little as she shuffled her feet out of the way, but continued holding him up.

“Doctor Gaster!” she yelled “Odin! Anyone!”

Sans wasn't quite sure what happened after that – everything went a little hazy, blending together: he heard the sound of hurried footsteps and worried shouting, followed by the sensation of being picked up, but after that... he felt like he was melting, his head swimming as his consciousness waned in and out.

* * *

 

Oh god, oh god, what was happening?! What was wrong with Sans? He hadn't... 'fallen down', had he?! He was still so young, and besides from being a little overweight, he was in perfect health. Alphys could barely concentrate, constantly glancing down the hall in hopes that Dr Gaster would reappear and assure her that everything was okay. Luckily the machine was working just fine – they had already filled a vial and a half with 'determination' without springing any leaks or overheating – but the project was the last thing on Alphys' mind.

She nearly leapt clear out of her skin when her wish was granted, Dr Gaster making some notes on a clipboard and handing it to Dr Odin, who followed behind him looking pale and forlorn.

“Dr Gaster!” Alphys cried, jumping down from the machine and running over “Sans! Is he okay?!”

“Alphys dear, don't do something so dangerous.” he scolded, straightening up her lab coat “You'll hurt yourself. And don't worry about Sans: he's just exhausted.”

“Exhausted?” she pried.

“Sans has really been going all out for our project.” Dr Gaster elaborated “Long hours here, the long commute back and forth to Snowdin, taking care of the children – it's no wonder he's worn himself down the bone.”

“I don't find that funny, Dr Gaster.”

“It was unintentional, I assure you.” he chuckled “Sans just needs a little rest. A few days in bed will do him the world of good.”

“What about the kids?” Alphys reminded him “We can take care of Frisk here, but Papyrus-!”

“I want you to pick Papyrus up from school.” Gaster told her “You can take care of him for a few days, can't you?”

“I...I guess... I mean, I don't really know him that well...”

“Of all of us, you're the closest to Sans, the one he trusts most.” he elaborated “I know you would be the one he would want to take care of his brother. He needs to know we have his back while he's out of service.”

“I guess...”

“And I'm certain.”

Gaster patted Alphys on the shoulder, smiling in that fatherly way of his as he encouraged her.

“We're about done here for the day.” he told her “We don't want to overexert the subject, after all. I'll shut everything down, so why don't you run along to Papyrus' school – if you go now, it's early enough to get some nice-cream on the way home.”

“Right... of course...” she agreed “Just... let me go change...”

* * *

 

Gaster watched as Alphys shuffled back down the stairs to her nest. Her shoulders were drooped as she dragged her feet – she was worried, obviously. She and Sans were close, after all... Dr Odin cleared his throat to get his attention, rubbing the back of his neck awkwardly.

“Is... Is Sans really okay?” he asked “I've never seen anything like this before.”

“We'll keep a close eye on him.” Dr Gaster assured “He's all hooked up to the monitors, if there's any changes we'll know right away.”

“What's wrong with him?” Dr Odin pried “I couldn't help but notice-”

“He's just exhausted.”

“But-”

“ _He's_. _Exhausted_.”

“... yes, sir...”

“Get back to work.”

“Yes, sir.”

Odin shuffled off. Gaster made good on his word and started shutting the extractor down, putting the vials safely in the containment unit. There was less than he hoped: they would have to turn up the settings tomorrow. Just as he finished, Alphys reappeared, dressed to go out with her overnight bag under her arm.

“I-I'm going to check on Sans before I go.” she announced “J-Just for my own sake.”

“Of course.” he smiled “I'll see you tomorrow.”

“F-Frisk likes lots of vegetables on their pizza.” she went on “Or else, there's some noodles in the cupboard – Frisk likes the beef ones.”

“Don't worry – I'll make sure they're fed.”

“Right... see you tomorrow.”

He watched her leave before packing up the containment unit and heading for the next room, pulling open the door of the closest fridge. The lights of the hallway flicked on automatically as he walked through, going black almost instantly as he passed. After a short walk, he descended the long staircase to the very lowest lab, placing the containment unit on the closest table.

The human was passed out, still strapped into the machine in the centre of the room. The tubes and pipes held firm, despite smoking slightly, Drs Hive and Head stood quietly at their posts making sure the shutdown went properly. Gaster glanced at the monitor as he passed – it was still alive, which was the main thing.

“Apologies for my absence.” he said to his colleagues “I take it everything went to plan.”

“U-um...” Dr Hive stammered quietly, not even able to look at him.

“Um?” Gaster repeated, tone harsh and uncompromising.

“F-fine.” he replied quickly “Everything went fine, Dr Gaster, sir. No problems.”

“Good. Dr Head?”

“Machines work perfectly.” he replied on the other side of the room, also not looking at him.

“Fantastic.”

Dr Gaster went back to the containment unit, feeling his colleagues eyes on him as he pulled out one of the vials.

“Was... was it supposed to be that way?” Dr Hive asked carefully “It hurt so much... the kid was... well...”

“Screaming.” Dr Head finished “Frisk was screaming. What kind of machine is this, Gaster?”

“The machine that's going to free us all.” he answered simply.

Gaster turned to face them, holding the glowing red vial for them to see.

“You know what this is?” he asked “This 'determination' – it's one tenth of what we need to free everyone in the underground. Gentlemen, we stand, today, closer than we have ever been in our lives, to freedom. In one day, we have achieved so much. In one day, we have bought monsterkind forward by thousands of years.”

The doctors exchanged a look. Gaster put the vial away properly, securing it in the machine where no harm could come to it.

“This is what Frisk wants.” Gaster reminded them “To help monsterkind. To free us.”

He turned back to them, putting his hands behind his back.

“We achieved so much in one day. In less than two weeks, we'll have enough 'determination' to break the barrier once and for all. Yes, it hurt the subject, but they're doing their best for everyone. We need to do our best and stay determined as well. For their sake. For the sake of our families. For the sake of the whole underground.”

The doctors looked at each other again. Gaster could see his words reaching them. The good doctor turned around, closing up the containment unit.

“I need to check on Doctor Odin's research.” he told them “If the subject wakes up, feed it.”

He picked up the case, going back up the stairs without a word.

 


	8. The Other Experiment

**Notes for the Chapter:**

> I got some great feedback on the last chapter! It really makes my day to read the comments and see people really thinking about what's going on in the story :) Please enjoy this next chapter!

With the final flick of the switch, the machine gently hummed to life. Alphys let out a sigh of relief, just happy they hadn't blown any fuses as the lights in the lab/Sans' utility room stayed on, and gave Papyrus a great big smile.

“Good job, assistant!” she beamed, holding up a spanner “Shall we give this bad boy a test run?”

Papyrus didn't smile – he hadn't in days – instead absently fiddling with the control dial in front of him. Alphys could understand his feelings: she wanted to mope and sulk as well, but she had to put on a brave face for the kid. It wasn't an easy thing to do – no wonder Sans had perfected that rictus grin of his.

Alphys walked forward, getting Papyrus' attention by placing the spanner in his hand.

“Why don't you do it?” she suggested “Let's try sending it forward by five minutes.”

Without so much as a flicker of his teeth, Papyrus set the dial, took the offered spanner and threw it haphazardly into the machine. It hung there a moment, suspended in the soft white light, before disintegrating down into particles and disappearing.

“Eeyyy, it's working!” Alphys squealed, pulling on Papyrus' sleeve “Isn't this exciting?!”

Again, Papyrus had no reaction, not even interested enough to pull his arm free. Wow... Alphys really wasn't good at this, and the sight of his unhappy face made her want to cry.

They were both startled by a sudden sharp knock on the door, Papyrus bolting for it like it held the answer to all his prayers.

“Sans?!”

As he opened it, his shoulders immediately drooped.

“Oh.” he sighed “You. Hi.”

Dr Gaster smiled warmly at him, ignoring his less than welcoming hello.

“Sorry to disappoint you, Papyrus.” he bid “But I did bring dinner. Good evening, Alphys dear.”

“Good evening, Dr Gaster.”

“Papyrus, why don't you go find us some plates and forks for this.” he suggested, handing the skeleton the bag of food he had bought with him “Alphys and I will be along in a moment.”

“Okay.” Papyrus mumbled, devastation that it wasn't his brother at the door clear in his voice.

Dr Gaster waited until he had left, closing the door behind him before he spoke again.

“I see you got your 'time machine' working.” he mentioned “It must have been difficult on your own.”

“Papyrus is a great assistant.” Alphys admitted, clearing up the bits of clutter that lay about “He's surprisingly smart, too – not as smart as Sans, but he takes instruction better. I-I figured I should keep him occupied, since he still doesn't want to go to school.”

“Still? It's been a week already. Have you gotten him to eat properly? Is he at least sleeping at night?”

“No, not well.”

“That's no good – he's going to get sick as well at this rate.”

“What about Frisk?” Alphys suggested “Papyrus might feel better to have at least one member of his family around.”

“Dr Odin is taking care of Frisk.” Gaster said plainly “Since his children are closer in age.”

“But I think-”

“Alphys,” Gaster interrupted, kneeling down and placing his hands on her shoulders, looking her sympathetically in the eyes “Do you really think you can handle taking care of _two_ children?”

The question hit Alphys on a sensitive nerve – he was right, of course, she could hardly handle Papyrus, but it still hurt. Dr Gaster was the last person to tell Alphys that she couldn't do something, so to hear it so earnestly right from his mouth...

“I-I guess...” she conceded “H-How is Frisk? Is the extraction going okay?”

“We're not getting quite the results we had hoped for,” Gaster admitted, standing and straightening out his clothes “But we're not giving up – it just may take a little longer than expected.”

“Oh, I see... I hope they don't feel too disheartened.”

“Everything's fine, I assure you.” he promised “You did such a good job on the DT extractor, there's been no issues with it at all.”

“That's good. What about... well... Sans? How is he? Any improvement?”

Gaster's face twitched. He wrung his hands uncomfortably, looking away from Alphys. Her heart stopped beating, clenching painfully in her chest. _Oh God_ , she silently prayed, _please say something._

“We...” Gaster started, slow like the words were painful to get out “We should prepare for the possibility that, well... that Sans won't wake up.”

Alphys let out a horrified gasp, covering her mouth to stop herself bursting into tears. This couldn't be happening! How?! Why?! He had been just fine such a short time ago! Alphys could feel herself start to shake, too many emotions rising up and overwhelming her, as Gaster put his arms around her.

“Alphys...” he cooed comfortingly “Alphys dear, please be strong. I know this is a difficult time, but Papyrus is going to need us: if Sans doesn't wake up – if he really has 'fallen down' – then we'll be the only family that child has.”

“I...I... y-yes.” Alphys sobbed, trying to force down the devastation that threatened to drown her “I-I-I can... S-S-S-Sans... W-what are we gonna d-do?”

“Try not to think about it.” Dr Gaster soothed, patting her head “If that time comes, I'll take care of everything. You don't need to worry: I'll take care of Papyrus as if he were my own son, For Sans' sake.”

Gaster wiped the tears from Alphys eyes, giving her another encouraging smile and allowing her to collect herself before ushering her into the house to eat. A few moments after they closed the door behind them, the machine flashed, a spanner falling unheard to the hard floor.

* * *

 

Sans knew something was wrong. He would have spikes of delirious fever, followed by uncontrollable shivering, every bone in his body feeling like it was made of ice. Concerning enough for anyone, but considering Sans was a skeleton, without the ability to even feel hot or cold in the first place... he was frightened. That was his prevailing thought. What was happening to him? During his lucid moments he thought of Frisk and Papyrus, and the experiments that were surely going on right now.

His lucidity was brief, however, and he soon lapsed back into delirium. He could just about remember the times he had woken up, conscious just long enough for Gaster to appear and force something down his throat, or for one of his colleagues to try and get him to eat, but he had no idea how long it had been. Had it been hours? Years? He honestly couldn't tell.

His eyes, his hands – they burned. His chest felt like it was ripping open. Oh, God... what was wrong with him? He had to get out of bed – he felt like he was melting into the sheets. He wordlessly called for Alphys, for Papyrus, for anyone, and begged with all his heart, but nobody came.

His nightmares were so vivid – was he plunged into darkness, or was the light blinding him like white-hot needles in his eyes? He could hear everything, all at once, everywhere, funnelled right into his ears, but curses on that terrible silence! He had to get up. He had to get up! He wasn't going to fall down! He couldn't! He had to get up!

* * *

 

It was very late. Everyone had gone home, leaving Dr Gaster to lock up the laboratory and skim through the data they had generated that day, which didn't take him long. The lab was so quiet at night, it was almost eerie, the only real sound the faint hum of the machines downstairs. With Alphys gone, there wasn't even the comforting far-off sound of her cartoons and tinkering. Gaster made himself a cup of tea, taking another look over the data to find any anomalies. He wasn't sure how long he was sat there in the quiet when an alarm on his phone started to beep, rousing him from his studies. He took a brief look at the screen before getting up and heading downstairs. 

Down in the observation room, Sans had woken up. In his efforts to get out of bed, he had fallen to the ground, panting and wheezing, clearly in pain as he tried to stand. Gaster watched him a moment: he still seemed... sticky. But he was determined to get to his feet. 'Determined'... Observation over, Gaster picked up a pair of gloves from the table and walked over, kneeling down beside his assistant.

“Sans?” he called “You're awake. How are you feeling?”

He responded by throwing up. Disgusting. Gaster patted Sans on the back, taking note of the panicked look on his face. There was a faint glowing in the back of his eye: interesting side effect – he'd have to watch that carefully.

“G-Gaster...” Sans spluttered, still spitting up mouthfuls of bile “Wha... what's wrong with me?!”

He descended into a coughing fit, whole body shaking like it was going to fall apart.

“You're not very well, Sans.” Gaster replied simply “Don't you remember? You've been working yourself too hard, spreading too thin. You need rest, my friend.”

“Pa-py... Fris...”

Sans threw up again. There was nothing left in his stomach, judging by the unnerving chalky sludge and small chunks of bone that he bought up. Gaster left his side a moment, going over to the sink and grabbing the ready glass.

“Don't worry, everything's taken care of.” he assured “Didn't I tell you before that you could count on us? We're taking care of the children.”

He turned on the tap, holding the glass near it for a moment, but Sans didn't seem cogent enough to notice. He turned the tap off and walked back over, kneeling down beside him with the glass.

“Time for your medicine.” he ordered “Drink up: you'll feel better.”

Sans winced painfully as he looked up, hardly able to hold his head under his own power.

“R-red?” he asked.

“It's medicine.” Gaster swore “I diluted it for you, since you keep throwing up. It's good for you.”

Sans looked like he really didn't want to: indeed, he tried once again to get up, using Gasters shoulder as a brace, but his strength failed him, causing him to fall practically into the doctors lap. Gaster took him firmly with his free arm, knowing he wasn't strong enough to resist, and tipped him back, forcing him to open his mouth.

“Now now, don't be difficult.” he cooed “I'm not doing this to hurt you.”

In one swift movement, Gaster poured the whole glass down Sans' throat. He started to cough and splutter, trying to throw the red liquid back up, so Gaster covered his mouth, holding his head back so that he had no choice but to swallow. Once he had, he started to cough again. Putting down the glass, Gaster held Sans tightly to stop him coughing himself to pieces. The glowing in his eye increased, as did his 'sweating'.

After a good few minutes of coughing, Sans settled down. His breathing was still laboured, painful and slow, settling only when his consciousness slipped back out of his grasp. Gaster picked him up easily, putting the little skeleton back to bed and hooking up the monitoring equipment again. Taking up the clipboard from the side of the bed, he made notes about the events, detailing the dosage and effects before putting it back. Turning around, he nearly leapt clear out of his skin to find Dr Odin stood in the doorway, looking ragged and judgemental.

“So you are dosing him with 'determination'.” he confirmed “I never agreed to help you do that. I never agreed to experiment on monsters.”

“Is that what you think this is?” Gaster challenged “Just some experiment? You think I would treat my own precious staff that way? My own family?”

“Yes.” Odin answered simply.

“Odin... my friend... that hurt. This isn't about _science_ , it isn't about experimenting or point scoring – it's about improving the lives of monsters.”

“Sans life doesn't seem particularly improved.”

Gaster stepped away from the bed, holding his hands up. Despite being taller than him, Dr Odin shrunk away as he stepped closer.

“Really think about it,” Gaster urged “This 'determination' is such a potent chemical, it possesses such raw power – just imagine the good it can do for monsters like Sans: the 1HP, the magically challenged, those who have 'fallen down'. By harnessing the power of 'determination', we can change the lives of all monsters in the underground for the better!”

“Whatever helps you sleep at night.” Odin grumbled.

“Don't be that way.” he went on, putting a reassuring hand on the other mans back “After all, unless I'm very much mistaken, doesn't one of your own children have 1HP?”

Odin didn't reply, but the look on his face spoke volumes, all of them very angry.

“If 'determination' could help them,” Gaster went on “As a father... wouldn't you do anything necessary?”

Gaster patted Odin on the back, ignoring his hate filled eyes, and herded him out of the room, flicking the light off to let Sans sleep in peace.

 

 

 


	9. The Other Other Experiment

**Notes for the Chapter:**

> Sorry for the delay in getting this one out - I got promoted at work and it's gotten very hectic! Anyway, please enjoy!

It was dark. Sans could see the blinking light of the security camera in the ceiling, blinking red every few seconds. Blink, blink, blink, blink. He wasn't sure how long he was staring at it before he realised it was there, but it felt like it was a while. The second thing he realised was that he could think in a straight line again – no more feverish cycling the same points over and over. Now that he could think, he started to remember: that red medicine Gaster kept pouring down his throat – since when had the Underground had something like that? That coffee Gaster had given him – had it been drugged? Why would Dr Gaster do that?

Frisk.

Dr Gaster needed Frisk for this experiments, but Sans had said no more. The fact that someone as enigmatic as Gaster couldn't simply talk Sans around spoke volumes of what kind of experiments he had planned. Then why not kill him outright? Have Sans out of the way for good, instead of playing this game of doctor?

Because he wanted to leave with everything – that was the only explanation Sans had. He wanted to experiment on Frisk, but still be the 'good guy', still have the respect of his researchers and his assistant and his lab, with 'Alphys dear' all safe and secure down in her nest. There's no way she'd want anything to do with him if she knew what he was doing. If Sans happened to get better, then the 'medicine' worked, and Gaster was a hero. If he didn't, then the doctor would have been seen to try his best against death itself and lost his assistant, his good friend, despite all his efforts.

Sans thought back to what Gaster had said before handing him the coffee – that speech about being family, about looking out for each other – it had all been part of the play, staged for Sans, or anyone who might be listening, or hell, even for Gaster himself. Frisk, after outliving their usefulness, was sure to meet some manner of 'accident', or some other 'strange illness' that Gaster could bravely fight a losing battle against.

Narcissistic jackass.

Getting out of that bed was the single hardest thing Sans had ever done: his bones felt ten times heavier than before, and he literally had to peel the sheets from his bare arms and legs, as if he had somehow partially fused with them. It hurt. He collapsed in a heap on the ground as he fell, still unable to see a damn thing. He was very weak, and very hungry. It took every ounce of strength he had to get up, using the bed as an anchor, every move and stumble soliciting groans of pain.

His movements triggered the overhead lights to turn on, and for the first time he realised where he was, but only because he had no damn idea. He had never seen this room before, and he knew every single inch of this place, or so he thought. It was clearly an observation room, but way more tricked out than the other one, with monitors of all kinds on every surface, a tangled mess of wires shoved anywhere that could be considered 'out of the way'. That being said, half the room also looked like storage, piled high with boxes, broken machines and reams of obsolete data.

And this was the place Gaster had chosen to shove Sans. Apt.

Wherever it was, there was only one way out – a long, long set of stairs, so long that it gave Sans vertigo just looking at them. His head went woozy, his knees weak. He had to get help, find Frisk and Papyrus and get them the hell away from Gaster. Like everything in Sans' life, no-one was going to come along and do it for him, so with a great amount of shuddering effort, he took the first step.

* * *

 

Gaster tapped his long, pale fingers on the desk erratically as he stared at the screen. The data was sound, his calculation flawless – why the hell wasn't he getting the results he wanted? No matter how much 'determination' they extracted from the subject, the amount of power it produced never exceeded that of a single soul. Whether it be one vial or one hundred vials, the amount of power remained constant. How? Why? There was no explanation for it.

“Dr Gaster?”

He stopped tapping, looking over at who had called him. Dr Hive looked at him sheepishly, like a child who had been caught doing something wrong.

“The king is on the phone.” he reported “He wants to know how our research is coming along.”

Oh good, just what he needed. Gaster sighed, rubbing his temples as he got up from his chair.

“Alright, I'll deal with him.” he conceded “Restart the extractor while I'm gone.”

“Yes, sir.”

Taking the steps two at a time, Gaster got the top just as the screaming started again, closing the door behind him so it wouldn't disturb his phone call.

* * *

 

Alphys had no desire to see someone die, but she just couldn't stay away any longer. She had to see Sans one more time, before he turned to dust. See his dumb bald head and his stupid grinning face and his awful stubby hands and... and... oh god, why was this happening? Why now? When everything was finally going well for them all? When he was finally happy? Alphys loved Sans a lot, he was by far her closest and most important friend, and she had seen the changes in him since Frisk had literally fallen into his life: his depression was easier, his smile kinder, his laugh lighter. He had been more genuine these days, not just going through the motions like he had been for so long, putting on a brave face.

Was there nothing she could do? Alphys would literally do anything to save him, and not just because the thought that she would have to become Papyrus' guardian terrified her. She _would_ do it, of course – she would never betray Sans by letting _those people_ get custody of him again – but still...

Under the guise of getting some fresh clothes, Alphys had made the trip back to Hotland, creeping silently into the lab. She knew Dr Gaster would send her away if he saw her, so she kept her head down, tip-toeing along the foyer to the lift. Up above, she could hear the man himself in a one-way conversation.

“Yes, your highness, everything's coming along quite well.” he promised “We're getting some very promising results and... tea? I'm afraid we're at a crucial stage right now, but I can pencil you in in a fortnight?”

Alphys slipped into the lift, happy that it ran almost silently. Once downstairs, she headed straight for the observation room, steeling herself for what she may see. However, Sans wasn't there. Alphys went cold. It wasn't too late already. It couldn't be. Gaster wouldn't be talking so happily on the phone if... Alphys turned tail, searching every room. Where would they have put him? Somewhere more comfortable? The break room? In one of the back rooms, to give him some privacy? No, he was nowhere. Her room, maybe? Even before she got to the bottoms of the stairs, she could tell he wasn't there.

It occurred to her that she hadn't seen anyone else, either. Where was everyone? Had Dr Gaster given them the day off? At a time like this? Had they moved Sans some place else? Determined to find someone, Alphys went back upstairs, going to the only room she hadn't checked. Just as she got there, an almighty crash came from... somewhere. It was loud enough that she could tell it was close, the ground shaking handily beneath her, but everything around remained intact. Where on earth had it come from? The foundations?

As she looked around, one of the fridges the lined the far wall fluttered open, a fine spray of dust floating out. What on earth? As she drew closer, she could tell it was plaster dust. From inside the fridge? Pulling the door open, behind the shelves full of chemicals, Alphys saw... a crack? A doorway? In a fridge? Her heart caught in her throat when she heard Sans' groan coming from within, tearing out the shelves without a second thought and proceeding inside.

* * *

 

Dr Odin hardly even looked around. He stirred his coffee disinterestedly as he gazed at the hole in his ceiling, tapping the spoon against the mug before he took a sip.

“I told Gaster those stairs weren't stable.” he noted “Hello Sans. How are you feeling?”

As much pain as he had been in before, Sans was in much worse now. Those rickety old stairs had given way when he was half way up, but instead of finding himself on top of a pile of boxes, he had somehow fallen through the ceiling of an entirely different room. It was darkly lit, and with his head already woozy, Sans could barely see a thing. Uncurling himself from the knotted mess he had landed in, he tried to hold himself up, barely managing.

“O-Odin...” he managed to gasp “Wh-where...?”

“'Where are we?'” he guessed “In the old part of the lab. It's was shut up even before you were born, so it's kind of falling apart. Y'know, I heard rumours that Dr Gaster used to do all sorts of sick experiments on other monsters down here. I never used to believe it, but... I guess they weren't just rumours.”

Odin sipped his coffee, turning back around to his computer to continue working. This wasn't like Odin – he was the first person to reach the first aid kit when someone was hurt, and for him to just ignore Sans... The skeleton tried again to get to his feet, grabbing the closest sturdy object to brace himself against as he stumbled through the rubble at his feet.

His hands slipped on the sheer surface, causing him to grab it. For being old equipment, it was in perfect condition, not giving an inch as it glowed gently. In fact, the only lights in the room were coming from the half a dozen large glass and metal cylinders and Odin's computer screen, making the place look more than a little ominous. What was Odin even doing down here? Trying to force himself to stand, Sans looked right into the glowing glass, feeling something unpleasant run down his back when he saw the small, dark mass that lay inside.

“D-Dr Odin...” he managed to say, ribs still aching from the fall “W-What did you say you were working on?”

“Echo flowers.” he replied robotically.

That thing was not an echo flower. The mass of cells, the distorted clump of something that couldn't be recognised, full of tubes and wires and floating in some manner of amniotic fluid, was definitely something of higher biology. Sans could identify a tiny dark spine, dark round eyes like a baby bird and the furious pulsations of a minuscule heart through the translucent skin. Looking around to the other tubes, he saw the same again, in various states of growth and mutation, and one that was straight up in pieces in its tube, the fluid having turned red.

Sans finally stood, glad for his flat feet as he leaned back against the tube.

“Odin.” he said without stammering “What are these things?”

“Humans.” a voice called from behind.

Dr Odin looked around for a second, but soon went back to his computer. With his usual gliding footsteps, hands poised elegantly in front of him, Dr Gaster appeared from behind Sans, walking between the tubes. Sans hated him on sight, especially as his face betrayed no emotion other than slight irritation. Stopping behind Dr Odin, Gaster turned to Sans, looking down at him with that same expression.

“This is project 'echo flower.'” he explained, gesturing to the tubes all around “Our 'plan B', should the extraction not work out the way he hoped.”

“You're growing humans?!” Sans demanded “How? Cloning?”

“Now now, Sans, you and I both know that cloned bodies don't develop souls.” Gaster went on “We needed to create something new. Luckily for us, there were already two humans in the underground – a 'mother' and a 'father' – so we didn't have to mess around splicing in monster genetics and just praying for a pure human soul to grow.”

“Y-You what?!” Sans gasped “What did you do to Frisk?!”

“Nothing.” the doctor assured “The whole process was achieved completely synthetically, thanks to the genius of our good friend Dr Odin here.”

Odin didn't look around as his name was called. His shoulders were slumped and unmoving as he fixed his gaze on the monitor.

“This is illegal.” Sans knew “And that's just the start of what's wrong with it!”

“I'm disappointed, Sans.” Gaster sighed, throwing up his hands “A man of your intelligence, of your vision, unable to see the bigger picture. These humans-”

“These THINGS are not HUMAN!” Sans roared, gesturing wildly at the tubes.

He felt his hand and eye start to burn. With an almighty crash of glass and liquid, one of them exploded, covering the floor in the sticky contents. It was this that made Odin finally look around from his computer, both him and Gaster looking at him in stunned amazement. Sans stumbled back, again using the tube behind him to brace himself as his head spun, his eye burning painfully. He was suddenly very tired. The delighted look on Gasters face didn't improve his mood. Gaster, however, soon composed himself, clearing his throat.

“Dr Odin, how many is that now?” he asked.

“Uh... we have five left.” Odin answered, looking at the tube full of floating pieces “If Alphys fixes the machine-”

“Alphys will have nothing to do with this.” Gaster said sternly, his tone heavily implying that he had said the same at least once before “We'll just have to wait until we can harvest one of the others, then use that tube.”

“Right...” Odin sighed.

“To answer your question, Sans.” he went on “It isn't important that they look human, just that they have human souls. It took some work – you can see the results of using too much determination in our dessicated friend here – but they're finally starting to develop. A few more months and they'll be ready to be 'born', and he can harvest their unique souls and use then to break the barrier.”

“That's messed up.” Sans retorted “Completely fucked... I can't believe the King and Queen agreed to a scam like this!”

“It isn't important how we get it done.” Gaster argued “Only that we get results. We're going to destroy the barrier, Sans, the King and Queen will be grateful. Their son will finally see the surface, be free and see the sky and the stars... don't you want Papyrus to be free, Sans?”

“Don't you bring him into this.” he growled “This isn't about the barrier any more – you've seriously crossed the line! Do you think people will honestly be okay with what you're doing? To know where those souls came from?!”

“And who are you to take away peoples hopes and dreams? As long as the barrier is gone, do you honestly think people will care how it happened? People will celebrate this achievement for years to come!”

“And when they find out you've been experimenting on other monsters? You think they'll celebrate that too?”

Gaster's smile finally faltered.

“What are you talking about?” he asked, although the wince in the corner of his eye gave him away “I've never-”

Sans responded by making another tube explode, not only splattering the fluid all over the walls, but tearing apart the biological blob that lay within. Gaster looked decidedly unhappy, but also thoughtful, choosing his words carefully. The actor was improvising.

“As expected of you.” he complimented “You always were such a fast learner.”

“YOU did this to me.” Sans growled “What did you do?!”

“Sans, I was only trying to help you,” Gaster insisted “You were very sick-”

“Because you poisoned me!”

“But the 'determination'-”

“ _Determination_?! Is that what you gave me?! Did you even know what it would do?!”

“It made you stronger! Did you ever think you were capable of magic like this?”

“I could've died!”

“You still might!” Gaster pointed out “Unless you let me help you! We need to stabilize you-”

“You're not coming anywhere near me!” Sans interrupted “You think I'm ever gonna trust you again? That _anyone_ is?”

“You'll fall apart, Sans.” Gaster warned “You'll melt. You'll die. You'll leave Papyrus all alone. Are you honestly okay with that? After you worked so hard to get him away from that family that rejected him?”

“I told you not to bring him into this!”

Gaster tilted his head to the side a little, standing upright as he thought. He straightened out his clothes.

“Fine.” he said finally “Fall apart. Become nothing more than a pile of sludge, locked away in a jar in a sealed room in a basement, unloved and forgotten in a decade. I'll hire a new assistant. I hear that Papyrus takes instruction pretty well. He'll never be as smart as you, of course, but with the right education, and the encouragement of his loving new father, I'm sure he'll make a fine lab-tech.”

The rest of Gaster's words descended into unintelligible murmurs in Sans ears. His hackles rose, something dark and ugly rearing up within him. Papyrus' loving new... what? He could not have heard that correctly... Gaster really did want to leave with everything, even things he had no business touching. Something rose not just within Sans, but behind him as well. He couldn't see it, but he knew it was big, and he knew it was _his_. Gaster didn't notice as he pontificated, but Odin did, getting up out of his chair and running right out of the way. A bright blue light started to shine behind him as his blaster charged, finally getting the good doctors attention.

“Oh.”

 


	10. The Truth

**Notes for the Chapter:**

> Sorry this one is a bit late - works been manic! Anyway, this is the last chapter, so steel your spines for some action!

Dr Odin looked around the ruined lab. Whatever that thing was that Sans had summoned, it had wrecked everything in one almighty flash of light. All the tubes were wrecked, their contents torn apart and burnt and unusable. The far wall had caved in completely, the floor torn up and ceiling hanging in dangerous swinging clumps above. He hoped it wouldn't fall on his head.

He also kind of hoped it would.

He never would've guessed Sans was capable of this kind of magic, this kind of violence, even dosed up with 'determination'. Then again, he had been learning a lot lately about what the monsters around him were capable of. Gaster was saying something to him, but he couldn't hear. The boss was excellent with magic, of course, and someone like Sans, who had never had much of it to speak of, wasn't going to kill him in one hit. He was hurt though, that was for sure. Was he still talking? Odin still couldn't hear him.

All he could hear was that screaming.

That was all he could hear for a while now.

With a desperate grimace, Gaster stopped trying to talk to him, practically shoving him out of the way to race up the stairs after Sans, leaving Odin alone in the rubble. He was vaguely aware of the door slamming far above.

Well... everything was in ruins now. Everything he had thrown himself away for. His work, his life, his standards, all scattered about his feet. Without thinking too hard about it, Dr Odin picked up a large shard of glass, cutting his fingers severely as he gripped it. Maybe it was time for him to test what he was really capable of.

He put it to his neck and pulled.

* * *

 

Sans burst through the door, slamming it closed behind him and using his whole body to keep it shut. His chest was heaving from having run up the stairs, but he couldn't relax. He couldn't dwell on what the hell he had just done, what that thing was that he had created. He had to find Frisk and get the hell out of here! He was certain they'd be in the old part of the lab, but since he had no idea how to get to it, he didn't even know where to start looking. As he was racking his brain, something hit the door behind him with great force, threatening to knock him off his feet.

“Sans!” Gaster yelled from the other side “Open this door! Right now!”

“Fuck you! Where's Frisk?!”

“Sans, you need help!”

Sans just screamed in frustration, doing his best to hold the door closed. To his side, one of the other fridges burst open, the other doctors rushing out. Upon seeing him, they froze.

“Sans!” Dr Hive cried, clearly ecstatic “You're awake!”

Dr Head, reading the situation better, stopped his friend from running forward to embrace him. It took Dr Hive a moment to realise what was really happening as Dr Gaster continued to pound against the door.

“Sans!” he screamed “Let me out of here!”

“You tell me where Frisk is!” Sans demanded again, not taking his eyes off the other two.

Dr Hive pointed instantly to the fridge they had emerged from, Dr Head pulling him back as Sans bolted for it. He heard the one behind him thrown open, but didn't look back, slamming the door closed as he ran. There was nothing in the long hallway to throw down and block the path behind him, so he just ran as fast as he could, trying to think up some desperate plan on the fly. He nearly fell down the stairs, he was running so fast, but managed to catch himself just before.

“Frisk?!”

Luckily, the lights were still on. What the hell was this machine?! It looked like something out of a nightmare, like a giant metal arm stretching down from the ceiling, hung up with wires and tubes and cables, reaching out to claw at whoever happened to be strapped to the chair below. There was no way Alphys would have built something like this.

“Frisk?!” he called again “Where are you, kid?!”

Sans heard something shuffle on the other side of the room. Spinning around, his heart stopped dead in his chest.

Frisk lay in a lump on the ground, barely conscious. The child was pale, very pale, the front of their shirt stained in what Sans desperately hoped was vomit. They were thin and dirty, hair matted from not being brushed... how long had Sans been unconscious? Sans sprinted over, afraid to even touch the child for a moment, before carefully laying a tentative hand on their shoulder.

Still breathing. Heart still beating. Thank God...

Frisk stirred, moaning slightly, but didn't look up, as if scared of who they'd see. Sans leaned over them, hand still on their shoulder.

“Frisk?” he called gently “Kid, you awake?”

Frisk startled, suddenly turning to him. The child looked at him like he was an angel descended from heaven, breathing heavily as they burst into tears.

“SANS!”

Frisk threw their arms around Sans' neck, screaming his name over and over again as they climbed into his lap. Sans held the child tightly, letting them scream it out, just happy that they were alive. Everything else could be fixed, as long as Frisk was alive. As his hand brushed something solid, he heard the dull 'clink' of heavy metal: he realised for the first time that Frisk was chained to the wall, heavy collar around their neck.

Well... Sans felt something dark rise up in him. Before he even realised it, the blaster had charged over his shoulder, firing with extreme prejudice at the wall. The whole thing disintegrated, carving a good few feet of tunnel into the rock and dirt behind before the giant floating skull disappeared. Sans still wasn't quite used to his magic, but as Dr Gaster said, he was a fast learner, turning his attention to the collar. It manipulated easily under his will, finally snapping clear away, making Frisk flinch. Sans made sure the child's neck wasn't bleeding anywhere before helping them stand.

“Okay, Kiddo, we gotta get out of here.” he told them “Can you walk on your own?”

Despite obviously feeling weak, Frisk nodded, doing their best to stand unaided, but clutching Sans' arm all the same.

A thought struck Sans: no-one was coming after him. Even in the state he was in, it didn't take him this long to get down that hallway, but he could hear no footsteps or doors slamming open. If no-one was coming after him, it could only mean there was no other way out of the room than the way he had come in. He had to think...

* * *

 

“Dr Gaster, you're injured!” Dr Hive pointed out “I'll get Dr Odin-”

“I'm fine.” Gaster interrupted, not taking his eyes off the fridge door “This is nothing. Sans isn't used to his magic yet, he couldn't deal me that much damage.”

“Sans'... magic?!” Dr Head clarified “Since when did Sans have magic?”

“It's a side-effect of the medication I've been giving him.” he confessed “The good news is, it worked, but Sans is unstable. He needs our help, before he hurts himself, or worse. He have to restrain him, no matter what.”

“H-he still has 1HP.” Dr Hive pointed out “It's gonna be hard not to hurt him.”

“Yes it is.” Gaster growled.

There was another explosion below, rocking the building as something collapsed. Was Sans trying to tunnel his way out? He'd wear himself out before he ever escaped. The three doctors startled as the fridge next to them was forced open, plaster and dust flying out in a great cloud. Gaster realised immediately: he was trying to blast through into the next room and escape that way.

“Dr Hive, get down there!” he ordered “Stop Sans, whatever it takes! Dr Head, take the opposite room, in case he's bluffing!”

They did as ordered, running off in opposite directions. Gaster grabbed the handle on the door in between, thinking he would surround Sans completely, only to have it suddenly open from the other side, smacking him painfully in the face. He was hit with such force that he was knocked off his feet, long legs getting kicked several times as Sans bolted past him, pulling Frisk behind.

“Wait!”

Gaster went to grab Frisk and pull them both back, only to have them both suddenly disappear, reappearing on the other side of the room. Sans stumbled, the look on his face betraying the fact that he had no idea that was going to happen, but he recovered quickly, giving Gaster a healthy glare as he pulled Frisk into a tight hug.

“Sans, don't!” Gaster begged.

It was too late – he was already gone.

* * *

 

“Sans... Sans, stop.” Frisk begged, practically in tears “H-hurts...”

Sans was bent double, sweating profusely, not able to catch his breath as he leaned against the moist wall beside the waterfall closest to Snowdin. His whole body hurt, shaking uncontrollably. He really, really wanted to throw up, but he hadn't eaten anything in so long, there was just nothing there. His bones weren't used to this kind of magic, he didn't know how to cope. Frisk looked like they wanted to hug him, but was too afraid.

“Hurts...” they sobbed again “S-stop...”

“It's okay, Frisk.” Sans lied “I'm okay. I'm not hurt.”

Frisk wasn't buying it. Knowing time was against them, Sans grabbed Frisk's hand and pulled them along the path. The cold air of Snowdin bit them both bitterly, but luckily the house was close. Not even thinking that the door might be locked, Sans pulled it open.

“Papyrus?!” he called into the house “Are you here?!”

Wait, what time what is? What day? Would he be at school? Sans whole being lurched: he almost collapsed, grabbing the sofa.

“Sans?!” Frisk cried.

“Frisk, go get changed.” Sans ordered “Brush your hair. We're going to see the King and Queen.”

Frisk looked confused, still shaking and upset. Seeing them about to start crying again, Sans smiled reassuringly, patting them on the head.

“It's okay.” he promised “Everything's gonna be okay. They're not gonna get away with what they've done. We're gonna tell the King and Queen, and Gaster is gonna go to jail for sure. I'm gonna get us something to eat, so go wash your face, okay?”

This seemed to calm the child down. They still looked concerned, but after a moment, nodded and ran up the stairs.

Sans desperately needed food. He went to the kitchen, hoping beyond hope that there was something edible in it. Before he could open the fridge, he spotted a note in Alphys' tell-tale chicken-scratch pinned to the front.

 

Gaster,

 

I decided to take Papyrus to the capital to see the puzzle museum – I thought maybe it would cheer him up. We'll stay there overnight, so don't worry about bringing us any more food. There's enough leftovers to feed an army! We'll be taking the river person's boat – sorry to make you have to walk.

 

See you later

 

Alphys.

 

PS. By the way, the machine works. Key is in ~~yo~~ Sans' room.

 

A

 

Alphys' handwriting was hard to read as ever, but that post-script... If they were tying up the river person with their trip, then it would take Gaster a while to get here. At least it was Alphys taking care of Papyrus and not...

Leftovers. Food. Sans tore open the door, half expecting to find another dark hallway behind it, but instead he was confronted with glorious boxes and containers of leftover meals. Something primal took over Sans, knocking out his good common sense as he tore open the first container and ate it right there, cold, no plate, no knife or fork, hardly even chewing as he shoved it down his gullet. By the time he realised what he was doing, he had eaten half the fridge, the empty boxes strewn about his feet.

“Sans?”

Startled, he looked around – Frisk was done changing, putting on their blue and pink-stripped sweater and brushing their hair. They were clearly confused, staring at Sans as he stood there with his hands in the fridge, face and robes covered with sauce. Sense kicking back in, he pulled out whatever was left and quickly arranged a few plates, piling them high, and shoved one in the microwave.

“That one's yours.” he informed them “Wait until it's hot before you eat it. If it's not enough for you, you can eat some of mine.”

“Okay...”

“I have to change.”

Sans ran from the kitchen, pulling off his robe as he went. He felt stronger now, if slightly unwell after eating it all cold. He went to the bathroom to wash the sauce off his face, the cold water clearing his head and giving him chance to think.

That note wasn't for Gaster – Alphys would've just texted him to tell him not to come over, especially if she knew he was going to bring food. Putting that aside, there was no puzzle museum in the capital. 'Puzzle museum'... wasn't that what she called her parents house? There was no doubt: Alphys had written that note for him, to let him know that Papyrus was safe. With Papyrus not here, there had been no reason for Sans' to come this way. Shit... with those two tying up the river person, there was only one way back, and he was bound to run into Gaster or one of the others on the path.

'The machine' was working? What machine? Face washed, Sans went to his room and pulled on some fresh clothes. Fed and washed, he started to feel like a person again, but he couldn't let himself relax just yet. The key was in his room? The key to where? Looking around, he saw the key to his utility room sat under his lamp, habitually shoving it in his pocket. The utility room... did Alphys mean that she got the time machine working?! Seriously?!

Sans was startled as Frisk burst into his room, hurriedly slamming the door and locking it behind them.

“Kid?!”

Frisk desperately shushed him, putting their hand to their lips. As Sans went silent, he heard the front door slamming open. Dammit – even without the river person, Gaster was a fast son of a bitch. Sans copied Frisk's gesture before silently pulling on his shoes and coat, pulling open the window. Frisk looked concerned, not wanting to climb out, but it was just another bluff by Sans, who took the child in a tight hug again.

“Just one more jump.” he promised “It won't hurt.”

Concentrating, he teleported, the two reappearing outside by the utility room door. He could still hear them going through his house, running up the stairs to find him. Peaking around the corner, just past the mailbox, he could see Dr Gaster stood outside, blocking not only the exit of the house, but also the only path out of Snowdin. Where could he hide? There was nothing but a dead-end in the other direction, even if they weren't spotted right away.

Thinking fast, Sans unlocked the door to the utility room, both of them slipping inside and locking it again behind them. Alphys said the machine was working: sure enough, there was a piece of paper on the dashboard with some instructions written on it. Could Sans go back and stop this mess from ever happening? He quickly read the note, Frisk clinging to him as they heard the sounds coming from the house getting angrier.

Damn – the machine only worked going forward, not back. In the house, they could hear what was unmistakably Gasters voice giving orders, followed by someone running back up the stairs. Oh hell – the note was still taped to the fridge! Sans only just had time to double check that the door was locked before the footsteps above returned: they knew the key was gone. Shit, shit, shit! No matter what, he couldn't let them find Frisk!

Sans didn't even think about it, turning to the machine as the footsteps thundered out of the house. If Alphys said this thing worked, Sans was just going to have to put his last remaining speck of faith in her. How far would be enough? Things would probably still be dangerous in a few hours, but longer than a few days might be dangerous... picking at random, he set the dial for a week. That HAD to be enough.

The machine started up, glowing a soft blue. Sans turned to Frisk. The child backed away, clearly terrified, and after everything they'd been through, Sans didn't blame them. He put on his best smile, knowing he wasn't at all convincing, but tried his best anyway.

“Frisk... kiddo...” he all but begged “I need you to get in the machine.”

“No!” they replied immediately.

Sans took their hand, pulling them into a hug. If this didn't work... If Alphys had made some miscalculation... either way, Frisk was going to be gone.

Someone tried to open the door, making Frisk flinch violently. Not believing they weren't in there, something heavy was thrown against it. That door wasn't particularly heavy – they were going to get it.

“Sans...” Frisk whimpered.

“Everything's gonna be okay.” he cooed, stroking their hair “I won't let them get you.”

“I'm scared.”

“I know. It's okay.”

Sans finally released them. He tried to stay calm, keeping that smile in place even as he could hear the wood start to splinter.

“This won't hurt.” he promised Frisk “And you'll see me before you know it.”

Frisk still didn't want to. As tears welled in their eyes, they gripped Sans' hand tightly, finally nodding. They walked forward, stepping into the machine, shaking a little as they were enveloped in the light. Sans let go of Frisk' hand, going to the control panel.

“Please don't leave me.” Frisk begged “I don't want to be alone.”

“You're never alone, sweetheart.” he promised “Just close your eyes. When you open them again, I'll be right here.”

Frisk put on a brave face, but the tears fell anyway. After taking a good long look at Sans, the child closed their eyes, shoulders tensing as they expected pain. Sans really hoped this wouldn't hurt... He pressed the dial to start the process, Frisk disappearing into the light.

The door burst open, Dr Head falling to the ground, having served his purpose as a battering ram. Gaster stood in the doorway, hands behind his back, as he surveyed the scene. He didn't smile. There was no hint of charisma on his face. He was just pissed. He walked into the room, stepping over Dr Head, eyes locked on Sans.

Similarly, Sans didn't look away from him. As the machine powered down, Sans spun the dial, making it impossible to tell how long Frisk had been sent forward. After a few steps, Gaster stopped in front of him. They stared each other down, Dr Head getting up as Dr Hive looked on, not knowing what to do.

“Well,” Dr Gaster said finally “You've done it. You've destroyed the best hope the Underground has of ever seeing Freedom.”

“Glad to be of service.” Sans growled.

“Still,” he went on “We still have a few vials of 'determination' to experiment with. I built the accelerated growth machines once, I can build them again. If need be, there is still one human in the Underground.”

“You mean the King and Queen's kid?” Sans knew “You think they're just gonna hand them over to you? Especially with what I'm gonna tell them?”

Gaster finally smiled. It was dark, manipulative, with a hint of malice on it's edges, and Sans felt like he was finally seeing his true smile.

“The King is an idiot.” he said “He'll believe anything I say. The Queen will be harder, but not impossible. Perhaps their child will suddenly get sick, and their dear old friend Dr. W.D. Gaster will be the only one able to help them.”

“What makes me think you've already thought this all out?”

“Hang on,” Dr Hive interrupted, appearing just behind Dr Gaster “Isn't that Dr Alphys' time machine? If it works, all we have to do it wait: Frisk will reappear soon enough!”

“No, I don't think it will.”

“'It'?” Sans picked up, feeling his hackles rise.

“Never let it be said that I am not a man of my word.” Gaster announced “It's been two weeks since you and I had our little chat, Sans. As I promised, I won't lay a single finger on Frisk.”

What's that? What was his angle? He didn't want Frisk? Dr Hive and Head looked just as confused. Sans backed away as Gaster stepped forward, running his hand over the machine. He looked Sans right in the eye, taking an important cable in his long pale fingers, and smiled.

He pulled the cable from the machine.

“NO!”

Sans ran forward, but it wasn't enough: using his magic, Gaster tore the machine apart in one fell movement, sending wires and computer chips flying across the room. Dr Hive and Dr Head ran for it, the remnants of the door flapping erratically in their wake. Not satisfied with that, Gaster tore the top off the dashboard, pulling the cables out and throwing them to the ground.

“That thing has been nothing but trouble.” he announced “And now it's gone, we can concentrate on the task at hand. My current experiment has yielded some very interesting results, and I'm keen to begin further testing.”

He turned to Sans, putting on his friendliest smile like nothing had happened.

“What do you say, Sans?” he bid “Shall we get to work? I want to see what kind of power you have in that magnificent blaster! And did I see you teleport before?”

Sans sunk to his knees. The machine... it lay in pieces all around him. They needed that. Without it, Frisk... Frisk... Gaster had torn it apart in seconds. It had taken them months to build, they couldn't make another one in a week. Frisk was... no... no no no NO NO NO NO NO!!

“Sans?” Gaster called again, kneeling down beside him “Don't cry over that thing, Sans, it's just one less burden for you to worry about. Come now, we need to get you back to the lab.”

A terrible calm came over Sans. Feeling his eye start to burn, he looked at Gaster. The doctor smiled at him, patting him on the back, and held out his other hand as if to help him up. After looking at him for a moment, Sans took his hand.

“Hey, boss,” he said.

“Yes, Sans?”

Sans gripped Gasters hand tightly. He smiled at him, making the doctor flinch.

“Wanna see me teleport?”

* * *

 

They re-materialised in the lower workings of the core. Gaster immediately jumped away from Sans, keeping a good distance between them as the skeleton got to his feet. Teleporting so far hurt like Sans had never known before, but he was too numb to feel it. Gaster stumbled to his feet, still backing away from his assistant as he looked all around: the walkway was too far above them for him to escape, the rock around them the only thing protecting them from the churning magma.

“Sans, what are you planning to do?” he demanded “You're new to magic, you don't have any chance of beating me!”

“Don't really have a plan.” Sans admitted.

All around him, bones started appearing in the ground, growing like macabre thorns on some vast and terrible vine. Gaster kept backing away, not believing his eyes. A chattering caught the doctors attention – looking up, he saw not one, not two, but four of the blasters floating above them, clicking their jaws. Sans could see on Gasters face – he knew he was in trouble.

“I got no idea what I'm doing.” Sans admitted “Finding out how all this works in gonna be fun.”

The blasters opened their mouths, charging their attacks. Gaster wasn't going down without a fight, raising his hands to defend himself, but Sans didn't give him the chance, firing all his blasters at once. Gaster easily dodged one, narrowly missing another, singeing his coat on the third and getting hit square in the chest by the last, throwing him back a good ten feet. Before he could get his bearings, massive bones shot out from the ground to impale him, his slim frame the only thing that prevented that fate.

“Damn.” Sans 'tsk'ed “Guess my aim is off.”

Sans raised his hand, lifting the bones off the ground, and pointed them like spears at Gaster.

“Wait, wait!” Gaster cried “It's my turn, isn't it? We have to take turns, that's how it works!”

“I don't recall agreeing to that.”

He fired the bones at Gaster, who only just got to his feet to sprint away. Just as he avoided the bones, another blaster appeared in front of him, firing almost immediately. Gaster collapsed like a ragdoll, letting the blast fly over him, close enough to burn the front of his shirt.

“For an old guy, you can really move.” Sans complimented “Guess I'll have to up my game.”

Sans focused his attention on Gaster himself, and with a wave of his hand, he was pulled into the air, making him swear loudly.

“Damn, boss, _language._ ”

Sans slammed him into the ground. He lifted him up, slamming him down again. He did this several more times, making himself laugh. He was enjoying this more than he should, but he was beyond caring. He slammed Gaster into the solid metal wall of the core, letting him drop to the ground on his own. Gaster let out a groan, trying to force himself up, but Sans wouldn't allow it, smacking him full force with a flurry of bones.

Sans was learning a lot about his magic. He only dealt 1 HP of damage with every hit, but they all added up, draining his health like poison. Luckily, Sans was more than happy to keep hitting him, and thanks to Gaster, he had more than enough determination to see it through.

His bone attack kicked up the dust all around them, blinding him for a second as Dr Gaster was lost in the haze. It was all the time Gaster needed to capitalise, long black tentacles shooting out of the cloud.

“Shit!”

Sans dodged, narrowly avoiding them, and jumped onto the head of one of his blasters. He learned very quickly that they couldn't take his weight for long, disappearing into powder after shooting a good twenty feet out of harms way. As the dust settled, Sans saw Dr Gaster on his feet, having shed his coat, the long tentacles emerging from his back.

“You're a tough fighter.” he complimented “I'm surpr... no, I'm not surprised. Despite your obvious physical weakness, your soul has always been made of something tough. I'm certain the 'determination' I gave you only amplified what was already there.”

Gasters tentacles went to the rocks and bones around him, picking them up.

“This is going to hurt me more than it's going to hurt you.” he said.

He threw them full force at Sans. Remembering he could, he teleported out of the way. Gaster seemed to know exactly where he was going to materialise, appearing seconds after him and grabbing him around the throat, lifting him clear off his feet.

“You're still no match for me, Mr 1HP.” Dr Gaster announced “Did you forget about that? One hit and you're dust.”

Sans formed a bone in his hand, smacking Gaster hard around the face with it. Gaster dropped him, swearing loudly as he clutched his face. Sans dodged out of his reach, needing a second to catch his breath. Magic was very tiring – he was draining quickly, the sweating starting again. He couldn't last much longer: he needed to finish this quickly.

Gaster finally stood straight, checking his hand as he released his face. He looked pissed.

“You're not yourself, Sans.” he said “You need to come to the lab: I'll find a way to fix you.”

“Pick a story and stick to it!” Sans yelled at him “One second you're 'good guy' Gaster, the next you're 'phycho' Gaster. Make your fucking mind up!”

Gaster glared at him, the two pacing around each other as they considered their next move.

“Even if you could stick to the script, you honestly believe I'm going to listen to a word to say ever again?” Sans went on “After everything you've done? The mask has slipped, I've seen the real you, everyone has! Me, the other doctors, we all know what you've done, and once I tell the King and Queen, your name will go down in history for the despicable creature that you really are!”

Gaster lunged for Sans. Panicked, Sans threw his hand up, tossing Gaster high into the air. That seemed to be exactly his plan, as he grabbed the handrail of the walkway with his tentacles.

“Shit!”

Gaster's tentacles seemed to be about as load-bearing as Sans' blasters, as the doctor quickly pulled himself up, only just able to grab the edge of the walkway when they disappeared, leaving his legs dangling helplessly below him.

* * *

 

Shit shit shit shit shit! Gaster wasn't blessed with great physical strength, and with his HP so low, he couldn't haul himself up over the edge, fingers desperately gripping any finger-hold they could in the walkway. How the hell was Sans so good at this?! He could feel himself being pulled viciously as Sans tried to force him back down.

The lift was close by. If Gaster could get to it, he could get to the castle before Sans, cook up some story. Asgore would definitely believe him, especially with the state he was in. Sans was in a rough way too, that was obvious, but he was young enough and mad enough to work through the burn. Gaster swore loudly as he lost his grip, only just grabbing the very edge, fingers nearly breaking as Sans continued to try and pull him down. One of his shoes slipped off, falling down the long drop and landing on the rock below, the momentum of the fall causing it to roll into the magma pit.

Gaster was struggling – he was too hurt and too tired to cast his magic, too weak to pull himself up. He had to think – quick!

He startled as he heard the clicking of claws on the walkway – someone was coming!

“H-Help!” he called “Please! I've fallen!”

His saviour had arrived! His heart leapt clear out his chest and into heaven itself, especially as a familiar yellow head appeared from around the corner.

“Alphys!” he cried “Alphys dear, please help me!”

Gaster was so flustered, he didn't notice that Alphys wasn't. Without so much as gasp at finding her boss hanging over the edge of the core by his fingertips, she walked towards him, clipboard under her arm.

“Dr Gaster.” she greeted “What are you doing here?”

“Sans has lost his mind!” he told her “He attacked me! Alphys, please, I'm slipping!”

Alphys stopped in front of him.

“I was just on my way to the castle.” she told him calmly, not even a hint of a stutter in her voice “I thought the King and Queen would be interested in our latest research.”

“W-what?”

Alphys dropped the clipboard by Gasters head. He recognised the pages upon it immediately: it was the notes he had written on Sans. Gaster felt a cold shiver run down his spine as he looked back at Alphys. Her face wore an expression so cold, it froze the very air around her.

Oh no... not Alphys too... not his dear Alphys...

“I never want to look back and think that I didn't kill you when I had the chance.” she said, lifting her foot.

She bought it down hard on his hand, causing him to let go. Gaster screamed, holding on as hard as he could with his one remaining grip. Sans was pulling him harder, trying to get him down. At this rate, he'd throw him right into the magma! Alphys stomped on Gaster's other hand, but somehow he managed to hold on. She dug the claws of her feet into his flesh.

“If there's anything left of the man I respected, you'll let go.” she growled at him.

She lifted her foot to stamp on his hand again.

* * *

 

Sans didn't know how or why Alphys was here, but he didn't care. Even from this vantage, he could see that she was making no moves to help Gaster. Thank god. Sans continued to pull – that bastard was coming down one way or another!

He stopped when he heard Alphys scream – Gaster had used whatever strength he had left to wrap one of his tentacles around her ankle, pulling her over the edge as he fell. Alphys was holding on for dear life: being much heavier and having a lower centre of gravity than Gaster, she was in a much worse position, claws digging into the metal desperately.

“SANS!” she screamed.

“Alphys!”

Sans was running on fumes – he didn't have the energy to go after both of them. In a snap decision, he used the last of his magic to teleport up onto the platform, grabbing Alphys under the arms and pulling her up with all his might. She wrapped one arm around him fiercely, using the other to try and claw her way up. Neither was physically strong, and Alphys was heavy: it took a few minutes before they had her safely on the platform, where she threw her arms around him, nearly hysterical. She let go of him almost instantly, looking back over the edge.

“Dr Gaster?!” she cried.

“Shit! Where is he?!”

What was left of his tentacle turned to dust on Alphys' leg. They both looked over the edge to the rocks below, but there was no sign of him. Where was he? Where the fuck was he?!

“Sans...”

Alphys pointed to the magma. Sans saw where she was pointing to just in time to see the dark spot dissipate as the ash spread out, consumed by the overwhelming heat. They watched it until it disappeared completely, leaving no trace that it had ever been there in the first place. They looked at each other, both physically exhausted, and after a moment, bumped foreheads. They were like that for a few minutes, just letting themselves rest and catch their breath, before Alphys struggled to her feet.

“Let's go, bonehead.”

“After you, noodle breath.”

* * *

 

“... and you say the old part of the laboratory can be reached by the fridges in the back room?” Gerson, the head of the royal guard, finished.

“Yes, sir.” Alphys confirmed “But it's in pretty bad shape. It'll be dangerous.”

“Thanks for the warning.” he said “We'll be careful. RG 01, RG 02, you're with me. Any harm comes to the Queen, you're dust. Undyne, watch these two.”

“Yes, sir!”

The guards, bar one, marched out. Rumours about some kind of explosion at the lab had already reached the castle, so when Sans and Alphys turned up injured and exhausted, the royal guard was ready to listen to what they had to say. The King had looked over the evidence Alphys had bought him, only understanding enough to know that Gaster had been experimenting on other monsters. The Queen, being the brains behind the throne, was the one to really take charge, going with the guards to gather evidence of what Gaster had done.

The King was useful in his own way, grabbing a couple of blankets for the pair and preparing them some hot, sweet tea and sandwiches, making conversation as they waited. He left when the little prince stumbled in, having awoken from some nightmare with tears in his eyes and looking for his parents. The King excused himself to put his son back to bed, bidding them goodbye.

“Damn,” Alphys whispered to Sans “I didn't know the King was such a stud. Yowza!”

“Don't look now, but that fish is totally checking you out.” Sans whispered back.

“Oh shush, she is not.” she giggled, suddenly startling “Oh hell, she is!”

Sans laughed, but it was hollow, exhaustion radiating from every bone. He didn't realise he had closed his eyes until he felt Alphys put her hand on his face, causing them to snap open again.

“You okay?” she asked, face a picture of concern.

“I'm tried.” he admitted “Really, really tried.”

“Go to sleep.” she urged him, pulling his head onto her shoulder “Just don't stay asleep for two weeks this time.”

“Week and a half it is.”

“Idiot.”

* * *

 

Down in the old lab, the Queen and the royal guard found all sorts of disturbing things, not least of which was a large pile of dust that everyone suspected was once Dr Odin. Dr Hive and Dr Head had disappeared entirely, not a single soul in the underground having seen neither hide nor hair of them. For the sake of their families, and the underground in general, the truth about what had happened was being kept from the general public. Sans didn't need the publicity of being 'the monster that was experimented on', so he didn't mind at all. He just wanted to go home and be with his family.

Papyrus was so pleased to see him, he nearly knocked him clear off his feet as he ran over to hug him. Sans swore that he had grown a little. As they walked back from Alphys' parents house to their own, Papyrus just kept talking, telling Sans about all the great new (or rather, old) puzzles her dad had taught him. Sans was just happy to hear his voice, letting him go on.

“You sure you want to stay with us?” Sans asked Alphys as she made up her bed on the sofa “It's pretty boring around here.”

“It's only for a few weeks.” she knew “Until I can get my stuff from the lab and find a new place. Besides, a couch in boring Snowdin is better than being stuck in the 'puzzle museum' listening to my dad go on about the glory days.”

“That's savage, cupcake.” Sans chuckled.

Upstairs, the bathroom door opened, Papyrus wandering out in his pyjamas.

“Brushed my teeth.” he announced sleepily “Can you read me a story?”

“Of course.” Sans agreed “I'll be up in ten minutes. Pick which one you want.”

“Okay.”

The young skeleton yawned widely as he went into his room. The other two watched until he closed the door behind him.

“What are you going to tell him?” Alphys asked quietly “About Frisk?”

“I don't know.” Sans admitted, feeling his heart pull heavily in his chest “Are you sure we can't fix the machine?”

“We can try, but...” Alphys sighed deeply, shoulders drooping as she looked away from him “Those components... they weren't exactly easy to find. Gaster really wrecked the whole thing... even if we find a way to fix it, get different parts or somehow find everything we need in the dump... it'll take way longer than the time we have. I'm sorry, Sans...”

She put a supportive hand on his arm, trying to smile. He understood where she was coming from, smiling back.

“Okay.” he said “I guess I just gotta do this, then. Lemme just grab him a class of milk.”

Sans did just that, hearing Alphys sigh several times. The stairs seemed to be much steeper than before, leaving Sans tired when he got to the top, hearing Alphys sigh again. In his room, Papyrus had already gotten into bed, flicking through 'Fluffy Bunny' as he waited for Sans, usual happy smile on his face.

How could he tell him? How could Sans explain? How could he physically form the words? He didn't know. He didn't know. Sans put the glass of milk down on the night stand, taking the book from his brother.

“Listen, bro, before we start,” he began, not able to look at him “I... I need to talk to you about something.”

“You're not going away again, are you?!” Papyrus panicked.

“I'm not, don't worry.” Sans swore “Not as long as I can help it.”

“Oh... good.” Papyrus sighed “I was worried there. So, what is it?”

“Well...” he said “I'm not going away, but... someone else has... Frisk... and... they won't be back...”

Papyrus just looked at him, confused.

“Who's Frisk?”

* * *

 

It wasn't just Papyrus who forgot. After a few days, Alphys started to forget about Frisk as well. The two theorised that it was a side effect of the machine, but with it in pieces they couldn't test it out. Feeling his own memories of the child slipping away from him, he stashed a few of their drawings in his lab, hiding the unfixable machine behind a curtain so he didn't have to look at it. Within a month, Frisk had been forgotten by everyone in the Underground, the truth of the events at the lab twisting around in the memories of the few who survived so that they made sense without the child presence, and life carried on.

In time, the King and Queen lost their young children to tragedy, the King declaring war on the humans and driving a wedge between him and his wife. Finally allowed back in the lab, Alphys finished her 'fridge' (who later called himself 'Mettaton'), earning her the position of royal scientist, but Sans just couldn't bring himself to go back, despite her offers of a job, instead taking up a position as a sentry. He was only able to step foot in the building again after the resets began, but that was another story altogether.

One day, Sans was sitting at his station at the exit to the ruins. It was quiet, as usual, as he woke up from his mid-morning nap, taking a glance at his watch. It was about the time the lady on the other side of the door appeared, so he stretched and got up, making his way over. On the way, he spotted someone on the path ahead: knowing there was nothing but a big, sealed door in that direction, he hid behind the trees to observe.

It was a human. A young one. God dammit ... now he had to keep that promise he made to the lady. There went his afternoon. Well, there was no time like the present, so he followed after them, catching up at the rickety little bridge.

“Human,” he greeted, making the child flinch “Don't you know how to greet a new friend? Turn around and shake my hand.”

The child turned around. Sans breath caught. He knew that face...

 

 

**Notes for the Chapter:**

> Well, there it was. I certainly hope you enjoyed it! Its been great to write something again. If you've made it this far, please don't forget to leave a comment on the way out!


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